Reapply "OvmfPkg/QemuVideoDxe: Helper functions for unaligned port I/O."

Reapply "OvmfPkg/QemuVideoDxe: Helper functions for unaligned port I/O."

This reverts commit 438ada5aa5a1174940795678c2dae07cde8f3869,
reapplying 05a5379458725234de8a05780fcb5da2c12680e4.

Note that the commit now being reverted is technically correct; the only
reason we're reverting it is because it should not have been pushed past
the Soft Feature Freeze for the edk2-stable201811 tag.

Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Julien Grall <julien.grall@linaro.org>
Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
Cc: yuchenlin <yuchenlin@synology.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1319
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: yuchenlin <yuchenlin@synology.com>