passthrough: give XEN_DOMCTL_test_assign_device more sane semantics

passthrough: give XEN_DOMCTL_test_assign_device more sane semantics

So far callers of the libxc interface passed in a domain ID which was
then ignored in the hypervisor. Instead, make the hypervisor honor it
(accepting DOMID_INVALID to obtain original behavior), allowing to
query whether a device can be assigned to a particular domain. Do this
by folding the assign and test-assign paths.

Drop XSM's test_assign_{,dt}device hooks as no longer being
individually useful.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>