OvmfPkg/PvScsiDxe: Setup requests and completions rings

OvmfPkg/PvScsiDxe: Setup requests and completions rings

These rings are shared memory buffers between host and device in which
a cyclic buffer is managed to send request descriptors from host to
device and receive completion descriptors from device to host.

Note that because device may be constrained by IOMMU or guest may be run
under AMD SEV, we make sure to map these rings to device by using
PciIo->Map().

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=2567
Signed-off-by: Liran Alon <liran.alon@oracle.com>
Message-Id: <20200328200100.60786-14-liran.alon@oracle.com>
Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>