For long running jobs (save, managed save, dump & live migrate)
virsh runs a background thread for executing the job and then
has the main thread catch Ctrl-C for graceful shutdown, as well
as displaying progress info.

The monitoring code is written using poll, with a pipe used
to get the completion status from the thread. Using a pipe
and poll is problematic for Windows portability. This rewrites
the code to use a GMainLoop instance for monitoring stdin and
doing progress updates. The use of a pipe is entirely eliminated,
instead there is just a shared variable between both threads
containing the job completion status.

No mutex locking is used because the background thread writes
to the variable only when the main loop is still running,
while the foreground thread only reads it after the main loop
has exited.
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>