qemu_agent: fix deadlock in qemuProcessHandleAgentEOF

qemu_agent: fix deadlock in qemuProcessHandleAgentEOF

If VM A is shutdown a by qemu agent at appoximately the same time
an agent EOF of VM A happened, there's a chance that deadlock may occur:

qemuProcessHandleAgentEOF in main thread
A)  priv->agent = NULL; //A happened before B

    //deadlock when we get agent lock which's held by worker thread
    qemuAgentClose(agent);

qemuDomainObjExitAgent called by qemuDomainShutdownFlags in worker thread
B)  hasRefs = virObjectUnref(priv->agent); // priv->agent is NULL,
                                           // return false
    if (hasRefs)
        virObjectUnlock(priv->agent); //agent lock will not be released here

In order to resolve, during EOF close the agent first, then set priv->agent
to NULL to fix the deadlock.

This essentially reverts commit id '1020a504'. It's also of note that commit
id '362d0477' notes a possible/rare deadlock similar to what was seen in
the monitor in commit id '25f582e3'. However, it seems interceding changes
including commit id 'd960d06f' should remove the deadlock issue.

With this change, if EOF is called:

    Get VM lock
    Check if !priv->agent || priv->beingDestroyed, then unlock VM
    Call qemuAgentClose
    Unlock VM

When qemuAgentClose is called
    Get Agent lock
    If Agent->fd open, close it
    Unlock Agent
    Unref Agent

qemuDomainObjEnterAgent
    Enter with VM lock
    Get Agent lock
    Increase Agent refcnt
    Unlock VM

After running agent command, calling qemuDomainObjExitAgent
    Enter with Agent lock
    Unref Agent
    If not last reference, unlock Agent
    Get VM lock

If we were in the middle of an EnterAgent, call Agent command, and
ExitAgent sequence and the EOF code is triggered, then the EOF code
can get the VM lock, make it's checks against !priv->agent ||
priv->beingDestroyed, and call qemuAgentClose. The CloseAgent
would wait to get agent lock. The other thread then will eventually
call ExitAgent, release the Agent lock and unref the Agent. Once
ExitAgent releases the Agent lock, AgentClose will get the Agent
Agent lock, close the fd, unlock the agent, and unref the agent.
The final unref would cause deletion of the agent.

Signed-off-by: Wang Yufei <james.wangyufei@huawei.com>
Reviewed-by: Ren Guannan <renguannan@huawei.com>