Add helper to populate vhost-user message regions

Add helper to populate vhost-user message regions

When setting vhost-user memory tables, memory region descriptors must be
copied from the vhost_dev struct to the vhost-user message. To avoid
duplicating code in setting the memory tables, we should use a helper to
populate this field. This change adds this helper.

Signed-off-by: Raphael Norwitz <[email protected]>
Message-Id: <1588533678[email protected]>
Reviewed-by: Michael S. Tsirkin <[email protected]>
Signed-off-by: Michael S. Tsirkin <[email protected]>
Reviewed-by: Marc-André Lureau <[email protected]>