So common functions used by both HVM and PV are factored out from
xen-all.c to xen-common.c.
Finally rename xen-all.c to xen-hvm.c, as those functions are only
useful to HVM guest.
Create *-stub files and modify Makefile.target to reflect the changes.
Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
