hw/ppc/Kconfig: e500 based machines require virtio-net-pci device

This fixes when configuring with CONFIG_PCI_DEVICES=n:

  $ qemu-system-ppc64 -bios /dev/null -M ppce500
  qemu-system-ppc64: Unsupported NIC model: virtio-net-pci

And:

  $ qemu-system-ppc64 -bios /dev/null -M mpc8544ds
  qemu-system-ppc64: Unsupported NIC model: virtio-net-pci

Fixes: 98bd1db
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20190316200818.8265-10-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/ppc/Kconfig

@@ -98,6 +98,7 @@ config MAC_NEWWORLD
 config E500
     bool
     imply AT24C
+    imply VIRTIO_PCI
     select ETSEC
     select OPENPIC
     select PLATFORM_BUS