OvmfPkg/IoMmuDxe: Provide an implementation for SetAttribute

A recent change to the PciIoMap() function now propagates the return code
from the IoMmu protocol SetAttribute() operation. The implementation of
this operation in OvmfPkg/IoMmuDxe/CcIoMmu.c returns EFI_UNSUPPORTED,
resulting in a failure to boot the guest.

Provide an implementation for SetAttribute() that validates the IoMmu
access method being requested against the IoMmu mapping operation.

Suggested-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Message-Id: <c0f9e95f557b601a045da015c1a97201e8aec2ab.1706634932.git.thomas.lendacky@amd.com>
Tested-by: Min Xu <min.m.xu@intel.com>
Reviewed-by: Min Xu <min.m.xu@intel.com>