virutil: Introduce virReadSCSIUniqueId

virutil: Introduce virReadSCSIUniqueId

Introduce a new function to read the current scsi_host entry and return
the value found in the 'unique_id' file.

Add a 'scsihosttest' test (similar to the fchosttest, but incorporating some
of the concepts of the mocked pci test library) in order to read the
unique_id file like would be found in the /sys/class/scsi_host tree.