vt-d: enhance the support of Interrupt Remapping EIM and x2APIC

vt-d: enhance the support of Interrupt Remapping EIM and x2APIC

1) Clear Interrupt Remapping(IR) unit's CFI (Compatibility Format
Interrupt) to enhance security;
2) Move the iommu_setup() ahead and put it before we begin to use
IOAPIC so we can make sure after we enable Interrupt Remapping, the
later IOAPIC (and MSI) initialization would setup IOAPIC RTEs (and
MSI) with remappable format;
3) Enable x2APIC only when all VT-d engines support IR with EIM
(Extended Interrupt Mode). EIM enables external devices to deliver
interrupts to logical processor with >8-bit APIC ID.

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>