runqemu-internal: get rid of annoying but harmless error message

If tap0 interface has been created by other users on the system, we would
get error messages when using runqemu. Error messages are like below.
     Acquiring lockfile for tap0...
     touch: cannot touch ‘/tmp/qemu-tap-locks/tap0.lock’: Permission denied
     /buildarea2/chenqi/poky/scripts/runqemu-internal: line 139: /tmp/qemu-tap-locks/tap0.lock: Permission denied
     flock: 8: Bad file descriptor

The system can still boot up because runqemu would try to create a new tap
interface. So the error message above is harmless, yet somewhat annoying.

This patch fixes the above problem. With this patch, the output would be as
follows.

Acquiring lockfile for tap0...
Acquiring lockfile for /tmp/qemu-tap-locks/tap0.lock failed
Setting up tap interface under sudo
Acquiring lockfile for tap1...
Running qemu-system-arm...

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>