storage: Add additional errors/checks for disk label

storage: Add additional errors/checks for disk label

Let's check to ensure we can find the Partition Table in the label
and that libvirt actually recognizes that type; otherwise, when we
go to read the partitions during a refresh operation we may not be
reading what we expect.

This will expand upon the types of errors or reason that a build
would fail, so we can create more direct error messages.