virtio: Provide version-specific variants of virtio PCI devices

Many of the current virtio-*-pci device types actually represent
3 different types of devices:
* virtio 1.0 non-transitional devices
* virtio 1.0 transitional devices
* virtio 0.9 ("legacy device" in virtio 1.0 terminology)

That would be just an annoyance if it didn't break our device/bus
compatibility QMP interfaces.  With these multi-purpose device
types, there's no way to tell management software that
transitional devices and legacy devices require a Conventional
PCI bus.

The multi-purpose device types would also prevent us from telling
management software what's the PCI vendor/device ID for them,
because their PCI IDs change at runtime depending on the bus
where they were plugged.

This patch adds separate device types for each of those virtio
device flavors:

- virtio-*-pci: the existing multi-purpose device types
  - Configurable using `disable-legacy` and `disable-modern`
    properties
  - Legacy driver support is automatically enabled/disabled
    depending on the bus where it is plugged
  - Supports Conventional PCI and PCI Express buses
    (but Conventional PCI is incompatible with
    disable-legacy=off)
  - Changes PCI vendor/device IDs at runtime
- virtio-*-pci-transitional: virtio-1.0 device supporting legacy drivers
  - Supports Conventional PCI buses only, because
    it has a PIO BAR
- virtio-*-pci-non-transitional: modern-only
  - Supports both Conventional PCI and PCI Express buses

The existing TYPE_* macros for these types will point to an
abstract base type, so existing casts in the code will keep
working for all variants.

A simple test script (tests/acceptance/virtio_version.py) is
included, to check if the new device types are equivalent to
using the `disable-legacy` and `disable-modern` options.

Acked-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

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