Replace the `ps ax | grep ...` call with pkill

Using the pipes has the nasty side effect of killing the management
command itself. Having a guest openbsd these are the processes killed:

        root@i5:~ # ps ax | grep openbsd0
        33916  2  Is+      0:00.01 csh -c /bin/sh /usr/local/etc/rc.d/vm onerun openbsd0
        33920  2  I+       0:00.01 /bin/sh /usr/local/etc/rc.d/vm onerun openbsd0
        33955  2  S+       0:13.20 bhyve: openbsd0 (bhyve)
        34114  3  S+       0:00.00 grep --color openbsd0

Killing the grep process stops the execution of the f_stop function
before cleaning up /dev/vmm and the tap interfaces.

       root@i5:~ # tmux ls
       openbsd0: 1 windows (created Fri Oct 31 01:14:20 2014) [159x69]
       root@i5:~ # service vm onestop openbsd0

       Stopping openbsd0

       Sending ACPI shutdown to openbsd0
       Terminated
       root@i5:~ # pgrep -lf bhyve
       root@i5:~ # ls /dev/vmm
       openbsd0

This issue was not present when running the stop directive without an
argument (service vm onestop).

After this patch, and using the pkill command I didn't notice any
leftovers after the stop command, and it didn't change the behavior of
`service vm onestop`, though I only have one guest on this host, and I
didn't try the loop.