block/rbd: Add support for ceph namespaces

Starting from ceph Nautilus, RBD has support for namespaces, allowing
for finer grain ACLs on images inside a pool, and tenant isolation.

In the rbd cli tool documentation, the new image-spec and snap-spec are :
 - [pool-name/[namespace-name/]]image-name
 - [pool-name/[namespace-name/]]image-name@snap-name

When using an non namespace's enabled qemu, it complains about not
finding the image called namespace-name/image-name, thus we only need to
parse the image once again to find if there is a '/' in its name, and if
there is, use what is before it as the name of the namespace to later
pass it to rados_ioctx_set_namespace.
rados_ioctx_set_namespace if called with en empty string or a null
pointer as the namespace parameters pretty much does nothing, as it then
defaults to the default namespace.

The namespace is extracted inside qemu_rbd_parse_filename, stored in the
qdict, and used in qemu_rbd_connect to make it work with both qemu-img,
and qemu itself.

Signed-off-by: Florian Florensa <fflorensa@online.net>
Message-Id: <20200110111513.321728-2-fflorensa@online.net>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

qapi/block-core.json

@@ -3544,6 +3544,8 @@
 #
 # @pool: Ceph pool name.
 #
+# @namespace: Rados namespace name in the Ceph pool. (Since 5.0)
+#
 # @image: Image name in the Ceph pool.
 #
 # @conf: path to Ceph configuration file.  Values
@@ -3570,6 +3572,7 @@
 ##
 { 'struct': 'BlockdevOptionsRbd',
   'data': { 'pool': 'str',
+            '*namespace': 'str',
             'image': 'str',
             '*conf': 'str',
             '*snapshot': 'str',