virtio-pci-modern: introduce helper to set config vector

stable inclusion
from stable-v5.12
commit 1a5c85f1
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5WXCZ
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=1a5c85f16594416df258fe38ec05e0dd05846479



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This patch introduces vp_modern_config_vector() for setting config
vector.

Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210104065503.199631-6-jasowang@redhat.com


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