scsi, file-posix: add support for persistent reservation management

It is a common requirement for virtual machine to send persistent
reservations, but this currently requires either running QEMU with
CAP_SYS_RAWIO, or using out-of-tree patches that let an unprivileged
QEMU bypass Linux's filter on SG_IO commands.

As an alternative mechanism, the next patches will introduce a
privileged helper to run persistent reservation commands without
expanding QEMU's attack surface unnecessarily.

The helper is invoked through a "pr-manager" QOM object, to which
file-posix.c passes SG_IO requests for PERSISTENT RESERVE OUT and
PERSISTENT RESERVE IN commands.  For example:

  $ qemu-system-x86_64
      -device virtio-scsi \
      -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
      -drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0
      -device scsi-block,drive=hd

or:

  $ qemu-system-x86_64
      -device virtio-scsi \
      -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
      -blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0
      -device scsi-block,drive=hd

Multiple pr-manager implementations are conceivable and possible, though
only one is implemented right now.  For example, a pr-manager could:

- talk directly to the multipath daemon from a privileged QEMU
  (i.e. QEMU links to libmpathpersist); this makes reservation work
  properly with multipath, but still requires CAP_SYS_RAWIO

- use the Linux IOC_PR_* ioctls (they require CAP_SYS_ADMIN though)

- more interestingly, implement reservations directly in QEMU
  through file system locks or a shared database (e.g. sqlite)

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

docs/pr-manager.rst

@@ -0,0 +1,51 @@
+======================================
+Persistent reservation managers
+======================================
+
+SCSI persistent Reservations allow restricting access to block devices
+to specific initiators in a shared storage setup.  When implementing
+clustering of virtual machines, it is a common requirement for virtual
+machines to send persistent reservation SCSI commands.  However,
+the operating system restricts sending these commands to unprivileged
+programs because incorrect usage can disrupt regular operation of the
+storage fabric.
+
+For this reason, QEMU's SCSI passthrough devices, ``scsi-block``
+and ``scsi-generic`` (both are only available on Linux) can delegate
+implementation of persistent reservations to a separate object,
+the "persistent reservation manager".  Only PERSISTENT RESERVE OUT and
+PERSISTENT RESERVE IN commands are passed to the persistent reservation
+manager object; other commands are processed by QEMU as usual.
+
+-----------------------------------------
+Defining a persistent reservation manager
+-----------------------------------------
+
+A persistent reservation manager is an instance of a subclass of the
+"pr-manager" QOM class.
+
+Right now only one subclass is defined, ``pr-manager-helper``, which
+forwards the commands to an external privileged helper program
+over Unix sockets.  The helper program only allows sending persistent
+reservation commands to devices for which QEMU has a file descriptor,
+so that QEMU will not be able to effect persistent reservations
+unless it has access to both the socket and the device.
+
+``pr-manager-helper`` has a single string property, ``path``, which
+accepts the path to the helper program's Unix socket.  For example,
+the following command line defines a ``pr-manager-helper`` object and
+attaches it to a SCSI passthrough device::
+
+      $ qemu-system-x86_64
+          -device virtio-scsi \
+          -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
+          -drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0
+          -device scsi-block,drive=hd
+
+Alternatively, using ``-blockdev``::
+
+      $ qemu-system-x86_64
+          -device virtio-scsi \
+          -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
+          -blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0
+          -device scsi-block,drive=hd