hv_xen: Fix issues with migration, add tests

Commit 3d942d8b broke instance migration (“self._cmd” was set to None).
This patch fixes that issue, refactors “MigrateInstance” for testing and
adds those tests.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>