iommu: add iommu_intpost to control VT-d Posted-Interrupts feature

iommu: add iommu_intpost to control VT-d Posted-Interrupts feature

VT-d Posted-Interrupts is an enhancement to CPU side Posted-Interrupt.
With VT-d Posted-Interrupts enabled, external interrupts from
direct-assigned devices can be delivered to guests without VMM
intervention when guest is running in non-root mode.

This patch adds variable 'iommu_intpost' to control whether enable VT-d
posted-interrupt or not in the generic IOMMU code.

Signed-off-by: Feng Wu <feng.wu@intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Jan Beulich <jbeulich@suse.com>