S390: QEMU driver support for CCW addresses

S390: QEMU driver support for CCW addresses

This commit adds the QEMU driver support for CCW addresses. The
current QEMU only allows virtio devices to be attached to the
CCW bus. We named the new capability indicating that support
QEMU_CAPS_VIRTIO_CCW accordingly.

The fact that CCW devices can only be assigned to domains with a
machine type of s390-ccw-virtio requires a few extra checks for
machine type in qemu_command.c on top of querying
QEMU_CAPS_VIRTIO_{CCW|S390}.

The majority of the new functions deals with CCW address generation
and management.

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