hvm: Fix 'xm save' bug in qhich qemu-dm ignores SIGTERM.

hvm: Fix 'xm save' bug in qhich qemu-dm ignores SIGTERM.

When saving a guest in early ChangeSet (Before 16750), Xend will
send "SIGKILL" signal to kill qemu process. But in ChangeSet 16750,
the signal is modified to "SIGTERM". However, "SIGTERM" handler has
been registered by SDL in qemu. So when qemu process receives a
"SIGTERM", SDL will catch it and it will push out a "SDL_QUIT" even to
qemu. If qemu can receive that event, it will be successfully
terminated. While we do "xm save", qemu can't go out from the dead
loop "while (suspend_requested) {...}" in main_loop() to handle the
SQL_QUIT event. So it is unable to poll that "SDL_QUIT" event. This
makes xm save command hang. This fix enables qemu to poll the
"SDL_QUIT" event while doing save.

Signed-off-by: Xu Dongxiao <dongxiao.xu@intel.com>