VT-d: Don't assume register-based invalidation is always supported

VT-d: Don't assume register-based invalidation is always supported

According to Intel VT-d SPEC rev3.3 Section 6.5, Register-based Invalidation
isn't supported by Intel VT-d version 6 and beyond.

This hardware change impacts following two scenarios: admin can disable
queued invalidation via 'qinval' cmdline and use register-based interface;
VT-d switches to register-based invalidation when queued invalidation needs
to be disabled, for example, during disabling x2apic or during system
suspension or after enabling queued invalidation fails.

To deal with this hardware change, if register-based invalidation isn't
supported, queued invalidation cannot be disabled through Xen cmdline; and
if queued invalidation has to be disabled temporarily in some scenarios,
VT-d won't switch to register-based interface but use some dummy functions
to catch errors in case there is any invalidation request issued when queued
invalidation is disabled.

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