qemu: Add support for RBD namespace.

qemu: Add support for RBD namespace.

Since Nautilus ceph supports separate image namespaces within a pool for
tenant isolation and QEMU adds it as a rbd blockdev options from 5.0.0.
The source name with format "<pool>/<namespace>/<image>" could be used to
access a rbd image with namespace.

Add unit tests for this attribute.

https://bugzilla.redhat.com/show_bug.cgi?id=1816909

Closes: https://gitlab.com/libvirt/libvirt/-/issues/405
Signed-off-by: Han Han <hhan@redhat.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>