librbd/api/Mirror: return EINVAL from image_resync()

librbd/api/Mirror: return EINVAL from image_resync()

... when mirroring is not enabled for the image.

Mirror::image_resync() returns ENOENT when mirroring is disabled for the
image. Instead, make it return EINVAL indicating that the call is
invalid when mirroring is not enabled for the image. This also causes
the public facing C, C++, and Python APIs that resync an image to
return EINVAL or raise an equivalent exception when mirroring is not
enabled for the image.

Signed-off-by: Ramana Raja <rraja@redhat.com>
(cherry picked from commit 4c992e6c8555ba370717c11d9d8ead1a52f97968)