tests: allow qemu-iotests to be run against nbd backend

To do this, we start a qemu-nbd process at _make_test_img and kill
it in _cleanup_test_img. $TEST_IMG is changed to point at the TCP
server. We also remove the checks for existence of binaries from
common.config - they're duplicated in common, and we can make the
qemu-nbd check conditional on $IMGPROTO being "nbd" if we do it there.

Signed-off-by: Nick Thomas <nick@bytemark.co.uk>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
