Not all device types were represented in virDomainDeviceType, so some
types of devices couldn't be represented in a virDomainDeviceDef
(which requires a different type of pointer in the union for each
different kind of device).

Since serial, parallel, channel, and console devices are all
virDomainChrDef, and the virDomainDeviceType is never used to produce
a string from the type (and only used in the other direction
internally to code, never to produce XML), I only added one "CHR"
type, which is associated with "virDomainChrDefPtr chr" in the union.