https://bugzilla.redhat.com/show_bug.cgi?id=1371892

The 'capacity' value (e.g. guest logical size) for a LUKS volume is
smaller than the 'physical' value of the file in the file system, so
we need to account for that.

When peeking at the encryption information about the volume add a fetch
of the payload_offset which is described as the offset to the start of
the volume data (in 512 byte sectors) in QEMU's QCryptoBlockLUKSHeader.

Then adjust the ->capacity appropriately when we determine that the
volume target encryption has a payload_offset value.