VT-d: centralize mapping of QI entries

VT-d: centralize mapping of QI entries

Introduce a helper function to reduce redundancy. Take the opportunity
to express the logic without using the somewhat odd QINVAL_ENTRY_ORDER.
Also take the opportunity to uniformly unmap after updating queue tail
and dropping the lock (like was done so far only by
queue_invalidate_context_sync()).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>