fix #2460: use dm-uuid-mpath- as fallback for multipath

fix #2460: use dm-uuid-mpath- as fallback for multipath

With Debian Buster it looks like the 'scsi-' method is no longer
reliable. In addition to that, which is also used for non-multipath
systems, add the 'dm-uuid-mpath-' method as fallback. This is also used
by openstack (see os-brick
https://github.com/openstack/os-brick/blob/39b201160bbff39117fca46b2709ff3f74c53d80/os_brick/initiator/linuxscsi.py#L400)

Also sort the output of readdir so 'scsi-' is always after
'dm-uuid-mpath-' so the output of pvesm list does not change for systems
that worked before.

Signed-off-by: Mira Limbeck <m.limbeck@proxmox.com>