disk: Fixup error handling path for devmapper when part_separator='yes'

disk: Fixup error handling path for devmapper when part_separator='yes'

https://bugzilla.redhat.com/show_bug.cgi?id=1346566

If libvirt_parthelper is erroneously told to append the partition
separator 'p' onto the generated output for a disk pool using device
mapper that has 'user_friendly_names' set to true, then the error
recovery path will fail to find volume resulting in the pool being
in an unusable state.

So, augment the documentation to provide the better hint that the
part_separator='yes' should be set when user_friendly_names are not
being used. Additionally, once we're in the error path where the
returned name doesn't match the expected partition name try to see
if the reason is because the 'p' was erroneosly added. If so alter
the about to be removed vol->target.path so that the DiskDeleteVol
code can find the partition that was created and remove it.