qemu: Unify generation of command line for virtio devices

qemu: Unify generation of command line for virtio devices

A virtio device such as

  <controller type='scsi' model='virtio-scsi'/>

will be translated to one of four different QEMU devices
based on the address type. This behavior is the same for
all virtio devices, but unfortunately we have separate
ad-hoc code dealing with each and every one of them: not
only this is pointless duplication, but it turns out
that most of that code is not robust against new address
types being introduced and some of it is outright buggy.

Introduce a new function, qemuBuildVirtioDevStr(), which
deals with the issue in a generic fashion, and rewrite
all existing code to use it.

This fixes a bunch of issues such as virtio-serial-pci
being used with virtio-mmio addresses and virtio-gpu
not being usable at all with virtio-mmio addresses.

It also introduces a couple of minor regressions,
namely no longer erroring out when attempting to
use virtio-balloon and virtio-input devices with
virtio-s390 addresses; that said, virtio-s390 has
been superseded by virtio-ccw such a long time ago
that recent QEMU releases have dropped support for
the former entirely, so re-implementing such
device-specific validation is not worth it.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>