xen: restrict: use xentoolcore_restrict_all

xen: restrict: use xentoolcore_restrict_all

And insist that it works.

Drop individual use of xendevicemodel_restrict and
xenforeignmemory_restrict.  These are not actually effective in this
version of qemu, because qemu has a large number of fds open onto
various Xen control devices.

The restriction arrangements are still not right, because the
restriction needs to be done very late - after qemu has opened all of
its control fds.

xentoolcore_restrict_all and xentoolcore.h are available in Xen 4.10
and later, only.  Provide a compatibility stub.  And drop the
compatibility stubs for the old functions.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Stefano Stabellini <sstabellini@kernel.org>