xenbus: Fix loopback event channel assuming domain 0

The xenbus event channel established in xenbus_init is intended to be a
loopback channel, but the remote domain was hardcoded to 0; this will
cause the channel to be unusable when xenstore is not being run in
domain 0.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Ian Campbell <Ian.Campbell@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>