Add virtio bus 'poll' method allowing us to inform backend we are
going to poll virtqueue.

Use on BERI soft-core to invalidate cpu caches.

Reviewed by:	bryanv
Sponsored by:	DARPA, AFRL
Increase the buffer size to keep the list of programm names when
parsing programm specification. It is safe to not check out of bounds
access, because !isprint(p[i]) check will stop reading, when '\0'
character will be read from the input string.

Obtained from:	Yandex LLC
MFC after:	1 week
Sponsored by:	Yandex LLC
Provide a dead version of if_get_counter.

Submitted by:	glebius
Reported by:	np