v2v: domainxml: Rework libvirt error handling.

Don't rely on libvirt's implicit print-to-stderr.  Instead make sure
that libvirt error messages are raised in the exception message.

Also allow error strings to be translated.
v2v: Allow domain UUID to be used in place of libvirt guest name (RHBZ#1141680).

Thanks: Zhou Junqin.
v2v: Allow pool UUID to be used in -os parameter (RHBZ#1141631).

Thanks: Zhou Junqin.