Revert "vm: Use virtio network driver."

This allows users to specify network interface settings with 'guix system vm'
without having to create a new NIC.  Fixes <https://bugs.gnu.org/42252>.
Reported by Christopher Lemmer Webber <cwebber@dustycloud.org>.

This reverts commit 5379392731.

gnu/system/vm.scm

@@ -819,7 +819,6 @@ with '-virtfs' options for the host file systems listed in SHARED-FS."
             '())
 
      "-no-reboot"
-     "-nic" "user,model=virtio-net-pci"
      "-object" "rng-random,filename=/dev/urandom,id=guixsd-vm-rng"
      "-device" "virtio-rng-pci,rng=guixsd-vm-rng"