lguest: fix ugly <NULL> in /proc/interrupts

Before:
	root@ubuntu:~# cat /proc/interrupts
	           CPU0
	  1:       1672    lguest-<NULL>    virtio0
	  2:          1    lguest-<NULL>    virtio1
	  ...
After:
	root@ubuntu:~# cat /proc/interrupts
	           CPU0
	  1:       2889    lguest-level     virtio0
	  2:          9    lguest-level     virtio1

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>