selftest/runqemu: Handle SystemExit

selftest/runqemu: Handle SystemExit

The sigchld handler in runqemu can raise a SystemExit when qemu shuts down.
Rather than backtracing, accept this as a successful test result.

ERROR: runqemu.QemuTest.test_qemu_can_shutdown (subunit.RemotedTestCase)
testtools.testresult.real._StringException: Traceback (most recent call last):
  File "/home/pokybuild/yocto-worker/nightly-oe-selftest/build/meta/lib/oeqa/selftest/cases/runqemu.py", line 183, in test_qemu_can_shutdown
    qemu_shutdown_succeeded = self._start_qemu_shutdown_check_if_shutdown_succeeded(qemu, shutdown_timeout)
  File "/home/pokybuild/yocto-worker/nightly-oe-selftest/build/meta/lib/oeqa/selftest/cases/runqemu.py", line 175, in _start_qemu_shutdown_check_if_shutdown_succeeded
    time.sleep(1)
  File "/home/pokybuild/yocto-worker/nightly-oe-selftest/build/meta/lib/oeqa/utils/qemurunner.py", line 100, in handleSIGCHLD
    raise SystemExit
SystemExit

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>