Installing the QEMU guest agent allows various other libvirt commands to become more powerful. The guest agent enhances the following virsh commands:
virsh shutdown --mode=agent - This shutdown method is more reliable than virsh shutdown --mode=acpi, as virsh shutdown used with the QEMU guest agent is guaranteed to shut down a cooperative guest in a clean state. If the agent is not present, libvirt has to instead rely on injecting an ACPI shutdown event, but some guests ignore that event and thus will not shut down.
Can be used with the same syntax for virsh reboot.
virsh snapshot-create --quiesce - Allows the guest to flush its I/O into a stable state before the snapshot is created, which allows use of the snapshot without having to perform a fsck or losing partial database transactions. The guest agent allows a high level of disk contents stability by providing guest co-operation.
virsh setvcpus --guest - Instructs the guest to take CPUs offline.
virsh dompmsuspend - Suspends a running guest gracefully using the guest operating system's power management functions.