OvmfPkg/VirtioLib: alloc VRING buffer with AllocateSharedPages()

OvmfPkg/VirtioLib: alloc VRING buffer with AllocateSharedPages()

The VRING buffer is a communication area between guest and hypervisor.
Allocate it using VIRTIO_DEVICE_PROTOCOL.AllocateSharedPages() so that
it can be mapped later with VirtioRingMap() for bi-directional access.

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: correct typo in VirtioRingInit() comment blocks]
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>