qemu: remove use of qemuDomainObjBeginJobWithAgent()

qemu: remove use of qemuDomainObjBeginJobWithAgent()

This function will be removed in a future commit because it allows the
caller to acquire both monitor and agent jobs at the same time. Holding
both job types creates a vulnerability to denial of service from a
malicious guest agent.

qemuDomainSetVcpusFlags() always passes NONE for either the monitor job
or the agent job (and thus is not vulnerable to the DoS), so we can
simply replace this function with the functions for acquiring the
appropriate type of job.

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