OvmfPkg/VirtioRngDxe: map host address to device address

patch maps the host address to a device address for buffers (including
rings, device specifc request and response pointed by vring descriptor,
and any further memory reference by those request and response).

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
[lersek@redhat.com: return EFI_DEVICE_ERROR if mapping fails in GetRNG]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>