Add a new storage pool source device attribute 'part_separator=[yes|no]'
in order to allow a 'disk' storage pool using a device mapper multipath
device to not add the "p" partition separator to the generated device
name when libvirt_parthelper is run.

This will allow libvirt to find device mapper multipath devices which were
configured in /etc/multipath.conf to use 'user_friendly_names' or custom
'alias' names for the LUN.