virtio-pci-modern: introduce helper to get notification offset

stable inclusion
from stable-v5.12
commit 1bfd8413
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1bfd84134c885799b8ac94766bff600b1f963ecf



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This patch introduces help to get notification offset of modern device.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210104065503.199631-15-jasowang@redhat.com


Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Pengyuan Zhao <zhaopengyuan@hisilicon.com>