migration: Add VMSTATE_UINTTL_TEST()

include/migration/cpu.h defines VMSTATE_UINTTL() and several variants
for migrating target_ulong fields.  It's defined in terms of
VMSTATE_UINT32() or VMSTATE_UINT64() as appropriate.

It doesn't, however, include a VMSTATE_UINTTL_TEST() variant, which
I'm going to need shortly.  So, add it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>