OvmfPkg/VirtioNetDxe: alloc RxBuf using AllocateSharedPages()

OvmfPkg/VirtioNetDxe: alloc RxBuf using AllocateSharedPages()

When device is behind the IOMMU, VirtioNetDxe is required to use the
device address in bus master operations. RxBuf is allocated using
AllocatePool() which returns the system physical address.

The patch uses VIRTIO_DEVICE_PROTOCOL.AllocateSharedPages() to allocate
the RxBuf and map with VirtioMapAllBytesInSharedBuffer() so that we can
obtain the device address for RxBuf.

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>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>
Tested-by: Laszlo Ersek <lersek@redhat.com>