i40evf: add client interface

In preparation for upcoming RDMA-capable hardware, add a client
interface to the VF driver. This is a slightly-simplified version
of the PF client interface, with the names changed to protect the
innocent.

Due to the nature of the VF<->PF interactions, the client interface
sometimes needs to call back into itself to pass messages. Because
of this, we can't use the coarse-grained locking like the PF's
client interface uses. Instead, we handle all client interactions
in a separate thread so the watchdog can still run and process
virtual channel messages.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Signed-off-by: Anjali Singhai Jain <anjali.singhai@intel.com>
Signed-off-by: Avinash Dayanand <avinash.dayanand@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>