Since e791519f0f ("nixos/qemu-vm: use qemu_kvm"), VMs generated with
nixos-rebuild build-vm use the qemu_kvm package instead of the qemu
package. (The difference between them is that qemu_kvm is only built
with support for the host architecture, not all architectures.)
But with this change, nixos-rebuild build-vm would now depend on
_both_ QEMUs, because the guest agent module was still using the one
from the full QEMU package. There's no need for it to use this
instead of the lighter qemu_kvm, because the guest agent shouldn't be
affected by which platforms QEMU can emulate.
Allow out of band communication between qemu VMs and the host.
Useful to retrieve IPs of VMs from the host (for instance when libvirt can't analyze
DHCP requests because VMs are configured with static addresses or when
there is connectivity default).