nixos/qemu-guest-agent: use qemu_kvm

Since e791519f0fb ("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.

nixos/modules/virtualisation/qemu-guest-agent.nix

···
       };
       package = mkOption {
         type = types.package;
-        default = pkgs.qemu.ga;
-        defaultText = literalExpression "pkgs.qemu.ga";
+        default = pkgs.qemu_kvm.ga;
+        defaultText = literalExpression "pkgs.qemu_kvm.ga";
         description = "The QEMU guest agent package.";
       };
   };