VT-d: improve RMRR validity checking

In order to make Xen more defensive to VT-d related BIOS issue, this
patch ignores a DRHD if all devices under its scope are not pci
discoverable, and regards a DRHD as invalid and then disable whole
VT-d if some devices under its scope are not pci discoverable. But if
iommu=force is set, it will enable all DRHDs reported by BIOS, to
avoid any security vulnerability with malicious s/s re-enabling
"supposed disabled" devices.  Pls note that we don't know the devices
under the "Include_all" DRHD are existent or not, because the scope of
"Include_all" DRHD won't enumerate common pci device, it only
enumerates I/OxAPIC and HPET devices.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
Signed-off-by: Weidong Han <weidong.han@intel.com>

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