virStorageSourceIsSameLocation: Special-case storage sources of type 'volume'

virStorageSourceIsSameLocation: Special-case storage sources of type 'volume'

The function is used also to compare virStorageSource which may not be
resolved to the image at that point in which case the 'path' is not yet
populated and the actual type is not yet set. This means that the
function fails to consider two identical volume-based disks as pointing
to the same thing.

Add a special case for both images being type=volume in which case we
compare only the pool/volume names.

Closes: https://gitlab.com/libvirt/libvirt/-/issues/240
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>