vm/qemu: use virtio-net-ccw as virtual netdev on s390x arch

virtio-net-ccw is a preferred way to set up a virtual network interface on
s390x at the moment because it is faster than virtio-net-pci
(eventfd and irqfd is missing). This also allows disabling of zPCI in QEMU
which was required only because virtio-net-pci was used as a network
interface. PCI is special on s390x and, for instance, does not use MMIO or
expose topology [1,2,3]. Furthermore, any features like PXE are
not supported with virtio-net-pci on s390x.

[1] https://people.redhat.com/~cohuck/2018/02/19/notes-on-pci-on-s390x.html
[2] https://wiki.qemu.org/Documentation/Platforms/S390X#A_note_on_PCI_support
[3] https://www.qemu.org/docs/master/system/s390x/pcidevices.html

Signed-off-by: Alexander Egorenkov <eaibmz@gmail.com>

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