VT-d PI: disable VT-d PI when CPU-side PI isn't enabled

VT-d PI: disable VT-d PI when CPU-side PI isn't enabled

From the context calling pi_desc_init(), we can conclude the current
implementation of VT-d PI depends on CPU-side PI. If we enable VT-d PI
and disable CPU-side PI by disabling APICv explicitly in xen boot
command line, we would get an assertion failure.

This patch clears iommu_intpost once finding CPU-side PI won't be enabled.
It is safe for this is done before this flag starts taking effect. Also
take this chance to remove the useless check of "acknowledge interrupt on
exit", which is a minimal requirement which has been checked earlier.

Signed-off-by: Chao Gao <chao.gao@intel.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Kevin Tian <kevin.tian@intel.com>
master commit: e489eb6138e7efe4214a7e9ba0d21f54fc5b7d35
master date: 2017-08-10 12:32:16 +0200