qemu-config: Add new -add-fd command line option

This option can be used for passing file descriptors on the
command line.  It mirrors the existing add-fd QMP command which
allows an fd to be passed to QEMU via SCM_RIGHTS and added to an
fd set.

This can be combined with commands such as -drive to link file
descriptors in an fd set to a drive:

    qemu-kvm -add-fd fd=3,set=2,opaque="rdwr:/path/to/file"
             -add-fd fd=4,set=2,opaque="rdonly:/path/to/file"
             -drive file=/dev/fdset/2,index=0,media=disk

This example adds dups of fds 3 and 4, and the accompanying opaque
strings to the fd set with ID=2.  qemu_open() already knows how
to handle a filename of this format.  qemu_open() searches the
corresponding fd set for an fd and when it finds a match, QEMU
goes on to use a dup of that fd just like it would have used an
fd that it opened itself.

Signed-off-by: Corey Bryant <coreyb@linux.vnet.ibm.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qemu-config.c

@@ -653,6 +653,27 @@ QemuOptsList qemu_boot_opts = {
     },
 };
 
+static QemuOptsList qemu_add_fd_opts = {
+    .name = "add-fd",
+    .head = QTAILQ_HEAD_INITIALIZER(qemu_add_fd_opts.head),
+    .desc = {
+        {
+            .name = "fd",
+            .type = QEMU_OPT_NUMBER,
+            .help = "file descriptor of which a duplicate is added to fd set",
+        },{
+            .name = "set",
+            .type = QEMU_OPT_NUMBER,
+            .help = "ID of the fd set to add fd to",
+        },{
+            .name = "opaque",
+            .type = QEMU_OPT_STRING,
+            .help = "free-form string used to describe fd",
+        },
+        { /* end of list */ }
+    },
+};
+
 static QemuOptsList *vm_config_groups[32] = {
     &qemu_drive_opts,
     &qemu_chardev_opts,
@@ -669,6 +690,7 @@ static QemuOptsList *vm_config_groups[32] = {
     &qemu_boot_opts,
     &qemu_iscsi_opts,
     &qemu_sandbox_opts,
+    &qemu_add_fd_opts,
     NULL,
 };