switch internal hypercall restart indication from -EAGAIN to -ERESTART

switch internal hypercall restart indication from -EAGAIN to -ERESTART

-EAGAIN being a return value we want to return to the actual caller in
a couple of cases makes this unsuitable for restart indication, and x86
already developed two cases where -EAGAIN could not be returned as
intended due to this (which is being fixed here at once).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com
Acked-by: Aravind Gopalakrishnan<Aravind.Gopalakrishnan@amd.com>
Reviewed-by: Tim Deegan <tim@xen.org>