When monitoring log output in livemedia-creator --no-virt it could get
stuck if the output from anaconda stops for some reason.
This changes execReadlines so that it will only read output when it is
available, will monitor the process state, and continue to call the
callback function.
It also adds a final timeout on proc.communicate() so that if Anaconda
becomes stuck and won't exit livemedia-creator will eventually exit.
When the no-virt callback terminates anaconda on an error it now sends a
TERM signal to all of the unshare process' children because just sending
it to unshare doesn't cause anaconda to exit.
(cherry picked from commit 6400515880)
