Fix multiple potential NULL pointer references in monitor usage
Any method which intends to invoke a monitor command must have a check for virDomainObjIsActive() before using the monitor to ensure that priv->mon != NULL. There is one subtle edge case in this though. If a method invokes multiple monitor commands, and calls qemuDomainObjExitMonitor() in between two of these commands then there is no guarentee that priv->mon != NULL anymore. This is because the QEMU process may exit or die at any time, and because qemuDomainObjEnterMonitor() releases the lock on virDomainObj, it is possible for the background thread to close the monitor handle and thus qemuDomainObjExitMonitor will release the last reference allowing priv->mon to become NULL. This affects several methods, most notably migration but also some hotplug methods. This patch takes a variety of approaches to solve the problem, depending on the particular usage scenario. Generally though it suffices to add an extra virDomainObjIsActive() check if qemuDomainObjExitMonitor() was called during the method. * src/qemu/qemu_driver.c: Fix multiple potential NULL pointer flaws in usage of the monitor
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