Hook front-panel ports to host netdevs

Link front-panel qemu nics to qemu netdevs.  Each front-panel port
creates a qemu nic with id <switch_name>.<port>.  A netdev can be
linked to the nic by id, either from the cmd line, or my using
shortcut switch backend propertry.  If using cmd line, invocation
example is:

   -rocker,name=sw1,fp_ports=2 -netdev tap,id=swp1.1,script=no,downscript=no
      -netdev tap,id=swp1.2,script=no,downscript=no

(front-panel ports are 1-based)

Using rocker property backend shortcut, the cmd line compacts:

   -rocker,name=sw1,backend=tap,fp_ports=2,script=no,downscript=no

In either case, the above examples would create nic-netdev linkage as:

(qemu) info network
hub 0
 \ user.0: index=0,type=user,net=10.0.2.0,restrict=off
 \ e1000.0: index=0,type=nic,model=e1000,macaddr=52:54:00:12:34:56
rocker.0: index=0,type=nic,model=rocker,macaddr=52:54:00:12:35:01
 \ sw1.1: index=0,type=tap,ifname=sw1.1,script=no,downscript=no
rocker.1: index=0,type=nic,model=rocker,macaddr=52:54:00:12:35:02
 \ sw1.2: index=0,type=tap,ifname=sw1.2,script=no,downscript=no

Shortcuts for other backends, like UDP sockets, can be added later.

Put front-panel support in it's own module.
rocker: add rocker prefix to register and bit names

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>
rocker: implement basic TEST functionality

Test registers and test IRQ.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: Scott Feldman <sfeldma@cumulusnetworks.com>