oeqa/utils/qemurunner: use the right kill call so we don't send SIGTERM to bitbake...

oeqa/utils/qemurunner: use the right kill call so we don't send SIGTERM to bitbake-worker

After recent bitbake-worker changes [1] this was killing the bitbake-worker,
which isn't what we want and kill(-pid) was the wrong call anyway.

runqemu.pid is the right PGID as the process was started with preexec_fn=setpgrp
(so no need to do os.getpgid(self.runqemu.pid))

[1] http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=15688798520896690561824b2fdc227c8a365c82

(From OE-Core rev: f4bf21b40ec6fa07c4a7bb450e3cfae12128dc2c)

Signed-off-by: Stefan Stanacar <stefanx.stanacar@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>