OvmfPkg/QemuFwCfgLib: Add option to dynamic alloc FW_CFG_DMA Access

OvmfPkg/QemuFwCfgLib: Add option to dynamic alloc FW_CFG_DMA Access

Update InternalQemuFwCfgDmaBytes() to work with DMA Access pointer.
The change provides the flexibility to dynamically allocate the "Access"
when SEV is enabled.

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