vtd: Move dom0 RMRR check to intel_iommu_remove_device()

vtd: Move dom0 RMRR check to intel_iommu_remove_device()

If put dom0 RMRR check in domain_context_unmap_one(), the devices with
RMRR cannot be assigned to other domain, becuase
domain_context_unmap_one() won't unmap context for them, and dom0
always owns them. This patch moves the check to intel_iommu_remove_device()
which is only called by dom0 hypercall.  This not only guarantees
keeping RMRR mappings for dom0 during its booting, but also won't
impact device assignment.

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