storage: Add helper to compare logical pool def against pvs output

storage: Add helper to compare logical pool def against pvs output

https://bugzilla.redhat.com/show_bug.cgi?id=1025230

Add a new helper virStorageBackendLogicalMatchPoolSource to compare the
pool's source name against the output from a 'pvs' command to list all
volume group physical volume data on the host.  In addition, compare the
pool's source device list against the particular volume group's device
list to ensure the source device(s) listed for the pool match what the
was listed for the volume group.

Then for pool startup or check API's we need to call this new API in
order to ensure that the pool we're about to start or declare active
during checkPool has a valid definition vs. the running host.