This patch adds the support for feature VIRTIO_PMEM_F_SHMEM_REGION
(virtio spec v1.2 section 5.19.5.2 [1]).

During feature negotiation, if VIRTIO_PMEM_F_SHMEM_REGION is offered
by the device, the driver looks for a shared memory region of id 0.
If it is found, this feature is understood. Otherwise, this feature
bit is cleared.

During probe, if VIRTIO_PMEM_F_SHMEM_REGION has been negotiated,
virtio pmem ignores the `start` and `size` fields in device config
and uses the physical address range of shared memory region 0.

[1] https://docs.oasis-open.org/virtio/virtio/v1.2/csd01/virtio-v1.2-csd01.html#x1-6480002Signed-off-by: Changyuan Lyu <changyuanl@google.com>
Message-Id: <20231220204906.566922-1-changyuanl@google.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>