ip/ipconfig: treat /32 mask as /0

some dhcp servers send a mask of 255.255.255.255 to indicate the gateway is directly routeable.

thanks to david du colombier for this patch.
ethervirtio: fix queue notifications and interrupt flags, avoid useless notifications

bug: Rnointerrupt was used on Vqueue.used.flags instead of
Vqueue.avail.flags.

introduce vqnotify() function that notifies the device
about available ring advancement.

avoid queue notifications there that can be slow by
checking Unonotify flag in Vqueue.used.flags.

keep track of the number of notifications in the queue.
merge