A RBD image can only be part of a single group. While an image is added
to a group, check if the image is already part of a group, and return an
error in case it is.
Signed-off-by: Niels de Vos <ndevos@ibm.com>
Add extra error checking to make sure trying to create an existing
volume group does not result in a failure. The same counts for deleting
a non-existing volume group, and adding/removing volumes to/from the
volume group.
Signed-off-by: Niels de Vos <ndevos@ibm.com>
