qemu/xen: make use of xenstore relative paths

Qemu has several hardcoded xenstore paths that are only valid on Dom0.
Attempts to launch a Qemu instance (to act as a userspace backend for
PV disks) will fail because Qemu is not able to access those paths
when running on a domain different than Dom0.

Instead make the xenstore paths relative to the domain where Qemu is
actually running.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Cc: xen-devel@lists.xenproject.org
Cc: Anthony PERARD <anthony.perard@citrix.com>
