OvmfPkg/VirtioNetDxe: log debug message in VirtioNetExitBoot()
    
    The other four virtio device drivers (VirtioBlkDxe, VirtioGpuDxe,
    VirtioRngDxe, VirtioScsiDxe) log such messages at this point; follow suit.
    
    Cc: Brijesh Singh <brijesh.singh@amd.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Contributed-under: TianoCore Contribution Agreement 1.1
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>