runqemu: check the qemu PID has been set before kill()ing it

If runqemu is killed, check that we have a valid PID for the qemu before
sending a kill() to it.

[ YOCTO #14651 ]

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 0f3afbd3)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>