targetcontrol: Fix leaking log handler

We had a mystery failure on the autobuilder where runqemu appeared to
be failing as a logfile directory no longer existed. The key to
reproducing was running a runqemu where the image was deleted (as
devtool does), then running another runqemu test. E.g.:

'oe-selftest -r  devtool.DevtoolExtractTests.test_devtool_deploy_target wic.Wic2.test_qemu_efi'

This then tries to write to the logfile from the first test, the
image directory was deleted and we get strange failures.

The fix is to remove the logging handler when qemu is stopped.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>