xen: Don't add dom0 twice on driver reload

xen: Don't add dom0 twice on driver reload

When the xen driver loads, it probes libxl for some info about dom0 and
adds it to the virDomainObjList. The driver then looks for any domains
in stateDir and if they are still alive adds them to the list as well.
This logic is a bit flawed wrt handling driver reload and causes the
following error

  internal error: unexpected domain Domain-0 already exists

A simple fix is to load all domains from stateDir first and then only
add dom0 if needed.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>