qemu: don't take agent and monitor job for shutdown

qemu: don't take agent and monitor job for shutdown

We have to assume that the guest agent may be malicious so we don't want
to allow any agent queries to block any other libvirt API. By holding
a monitor job while we're querying the agent, we open ourselves up to a
DoS.  So split the function into separate parts: one that does the agent
shutdown and one that does the monitor shutdown. Each part holds only a
job of the appropriate type.

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>