pybind/mgr/rbd_support: check whether mirroring is enabled

pybind/mgr/rbd_support: check whether mirroring is enabled

... before fetching the mirroring mode of the image.

In the CreateSnapshotRequests class, which asynchronously issues mirror
snapshot creation requests, prevalidation includes checking that the
image is enabled for snapshot-based mirroring and is marked as primary.
Since mirroring mode can only be queried if mirroring is already
enabled, the code first fetches the image’s mirroring info to verify
that mirroring is enabled, and only then retrieves the mirroring mode.

Signed-off-by: Ramana Raja <rraja@redhat.com>