get a handle for the tap device to shut it down

[Olaf Hering <olaf@aepfle.de>]

Xen does a shutdown of the emulated PCI network device in
pci_unplug_nics. But this just disables the PCI device. The tap device
for a given emulated card remains active because nothing closes the
file descriptor. 

The cmdline for qemu contains something like "-device
rtl8139,id=nic0,netdev=net0,mac=00:16:3e:28:f1:ee -netdev
type=tap,id=net0,ifname=vif1.0-emu,script=no,downscript=no".

I wonder if the missing disable of the tap device is intentional, or
just an oversight, or if its just to complicated to get from a
"PCIDevice *" to the other end and call the ->cleanup function.


Olaf