system: vm: Remove -net user flag.

* gnu/system/vm.scm (common-qemu-options): Remove -net user flag.

gnu/system/vm.scm

@@ -450,7 +450,6 @@ (define (virtfs-option fs)
            "")
      " -no-reboot -net nic,model=virtio \
   " #$@(map virtfs-option shared-fs) " \
-  -net user \
   -vga std \
   -drive file=" #$image
   ",if=virtio,cache=writeback,werror=report,readonly \