add-domain: Suppress libvirt errors on stderr.

add-domain: Suppress libvirt errors on stderr.

Install an error handler on the libvirt error connection so that
errors are not printed on stderr (instead they go up through the usual
libguestfs error mechanism).

Unfortunately this doesn't suppress initial connection error messages
to stderr.  I cannot see how to do this without affecting the global
libvirt error handler, which is not acceptable for a library to be
doing.