VT-d: improve RMRR validity checking

VT-d: improve RMRR validity checking

Currently, Xen checks RMRR range and disables VT-d if RMRR range is
set incorrectly in BIOS rigorously. But, actually we can ignore the
RMRR if the device under its scope are not pci discoverable, because
the RMRR won't be used by non-existed or disabled devices.

This patch ignores the RMRR if the device under its scope are not pci
discoverable, and only checks the validity of RMRRs that are actually
used. In order to avoid duplicate pci device detection code, this
patch defines a function pci_device_detect for it.

Signed-off-by: Weidong Han <weidong.han@intel.com>