vdpa: allow provisioning device features

vdpa: allow provisioning device features

This patch allows device features to be provisioned via vdpa. This
will be useful for preserving migration compatibility between source
and destination:

# vdpa dev add name dev1 mgmtdev pci/0000:02:00.0 device_features 0x300020000
# vdpa dev config show dev1
# dev1: mac 52:54:00:12:34:56 link up link_announce false mtu 65535
      negotiated_features CTRL_VQ VERSION_1 ACCESS_PLATFORM

Signed-off-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>