storage: Allow creation of a LUKS using logical volume
https://bugzilla.redhat.com/show_bug.cgi?id=1427049
Use virStorageBackendCreateVolUsingQemuImg to apply the LUKS information
to the logical volume just created. As part of the processing of the
lvcreate command add 2MB to the capacity to account for the LUKS header
when it's determined that the volume desires to use encryption.