OvmfPkg: Add PlatformHasIoMmuLib

Add the shorter-term library instance outlined in the previous patch to
OvmfPkg, so that we can imbue PciHostBridgeDxe with a protocol dependency
on gEdkiiIoMmuProtocolGuid OR gIoMmuAbsentProtocolGuid.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>