oeqa/qemurunner: do not use Popen.poll() when terminating runqemu with a signal

oeqa/qemurunner: do not use Popen.poll() when terminating runqemu with a signal

This does not actually guarantee that the child runqemu process has completely exited:
poll() may return prematurely while the SIGTERM handler in runqemu is still running.
This thwarts the rest of the processing, and may terminate the handler before
it completes.

Use Popen.communicate() instead: this is what python documentation recommends as well:
https://docs.python.org/3/library/subprocess.html#subprocess.Popen.communicate

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>