OvmfPkg/QemuFwCfg: introduce FW_CFG_IO_DMA_ADDRESS, adapt the package

OvmfPkg/QemuFwCfg: introduce FW_CFG_IO_DMA_ADDRESS, adapt the package

Introduce the FW_CFG_IO_DMA_ADDRESS macro for IO Ports 0x514 and 0x518
(most significant and least significant halves of the DMA Address
Register, respectively), and update all references in OvmfPkg.

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