cxgb4vf: Quiesce Virtual Interfaces on shutdown ...

When a Virtual Machine is rebooted, KVM currently fails to issue a Function
Level Reset against any "Attached PCI Devices" (AKA "PCI Passthrough").  In
addition to leaving the attached device in a random state in the next booted
kernel (which sort of violates the entire idea of a reboot reseting hardware
state), this leaves our peer thinking that the link is still up.  (Note that
a bug has been filed with the KVM folks, #25332, but there's been no
response on that as of yet.)  So, we add a "->shutdown()" method for the
Virtual Function PCI Device to handle administrative shutdowns like a
reboot.

Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>