Use a more specific error reporting invalid disk hardware

Instead of using NovaException for reporting invalid user
specified disk hardware, use UnsupportedHardware and
UnsupportedVirtType exceptions.

Change-Id: I68ed1c0569bd0c0679e9fff3ffd436197f78a9f0
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Merge "Use a more specific error reporting invalid disk hardware"