vdpa: Introduce and use vdpa device get, set config helpers

Subsequent patches enable get and set configuration either
via management device or via vdpa device' config ops.

This requires synchronization between multiple callers to get and set
config callbacks. Features setting also influence the layout of the
configuration fields endianness.

To avoid exposing synchronization primitives to callers, introduce
helper for setting the configuration and use it.

Signed-off-by: Parav Pandit <parav@nvidia.com>
Reviewed-by: Eli Cohen <elic@nvidia.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20211026175519.87795-2-parav@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>