xenbus: don't rely on xen_initial_domain to detect local xenstore

The xenstore daemon does not have to run in the xen initial domain;
however, Linux currently uses xen_initial_domain to test if a loopback
event channel should be used instead of the event channel provided in
Xen's start_info structure. Instead, if the event channel passed in the
start_info structure is not valid, assume that this domain will run
xenstored locally and set up the event channel.

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>