qemurunner.py: fix is_alive() to avoid confusing with recycled pid

qemurunner.py: fix is_alive() to avoid confusing with recycled pid

[YOCTO #12493]

Fixed:
- qemu started with pid 10000
- qemu exited unexpectedly
- The pid 10000 is re-used by another different process.

The is_alive() returned True in such a case because both qemu_pidfile and
/proc/10000 exist, but it's another process, this patch fixed the problem.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>