VT-d: improve RMRR region handling

VT-d: improve RMRR region handling

This patch improves RMRR regions handling as follows:

1) Get rid of duplicated RMRR mapping: different devices may share the
same RMRR regions, when they are assigned to the same guest, it only
need to map the same RMRR region once because RMRR region must be
identity mapped. Add an array of mapped RMRRs to achieve this.

2) Needn't call domain_context_mapping to map the device again in
iommu_prepare_rmrr_dev, and change iommu_prepare_rmrr_dev to
rmrr_identity_mapping which is more suitable.

3) A device may have more than one RMRR regions, remove "break" in
intel_iommu_add_device to let it map all RMRR regions of the device.

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