fence_scsi: Automatically detect devices for shared VGs

Currently, if no devices option is given, fence_scsi automatically
builds a device list containing all LVM PVs that back VGs with the
clustered ('c') bit set. With this commit, fence_scsi will also consider
VGs with the shared ('s') bit set.

Additionally, the existing check is too broad. We should consider a
volume group to be clustered or shared only if the 6th bit is set to 'c'
or 's'. This way, we can avoid false positives.

Closes RHBZ#2187327
Closes RHBZ#2187329