ArmVirtPkg, OvmfPkg: QemuFwCfgLib: move DMA-related defs to lib class

ArmVirtPkg, OvmfPkg: QemuFwCfgLib: move DMA-related defs to lib class

Move the type and macro definitions related to QEMU's DMA-like fw_cfg
access method to the library class header.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Leif Lindholm <leif.lindholm@linaro.org>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>