The qemu-guest-agent is a helper daemon, which is installed in the guest.
The qemu-guest-agent is used for mainly three things:
To properly shutdown the guest, instead of relying on ACPI commands or windows policies.
To freeze the guest file system when making a backup/snapshot (on windows, use the volume shadow copy service VSS).
In the phase when the guest (VM) is resumed after pause (for example after shapshot) it immediately synchronizes its time with the hypervisor using qemu-guest-agent (as first step).
Install Qemu-guest-agent