rbd: improve mirror image status and validation error messages

rbd: improve mirror image status and validation error messages

When a mirror image is left in a transitional state such as DISABLING,
the current mirror image status command reports:

  $ rbd mirror image status test_pool/test_image1
  rbd: mirroring not enabled on the image

This is the same message shown when mirroring is disabled or not yet
enabled, which can give the impression that mirroring is already
disabled.

Improve the validation logic and error messages to distinguish between
the DISABLED state and other non-enabled states, and include the image
name and current state in the output.

Examples:

When the image is completely disabled:

  $ rbd mirror image status test_pool/test_image1
  rbd: mirroring disabled on image 'test_image1'

When the image is in a transitional state (ex: DISABLING):

  $ rbd mirror image status test_pool/test_image1
  rbd: mirroring not enabled on image 'test_image1' (state: disabling)

Signed-off-by: Prasanna Kumar Kalever <prasanna.kalever@redhat.com>