S390: Add hotplug support for s390 virtio devices

S390: Add hotplug support for s390 virtio devices

We didn't yet expose the virtio device attach and detach functionality
for s390 domains as the device hotplug was very limited with the old
virtio-s390 bus. With the CCW bus there's full hotplug support for
virtio devices in QEMU, so we are adding this to libvirt too.

Since the virtio hotplug isn't limited to PCI anymore, we change the
function names from xxxPCIyyy to xxxVirtioyyy, where we handle all
three virtio bus types.

Signed-off-by: J.B. Joret <jb@linux.vnet.ibm.com>
Signed-off-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>