This patch refactors various places to allow removing of the
defaultConsoleTargetType callback from the virCaps structure.

A new console character device target type is introduced -
VIR_DOMAIN_CHR_CONSOLE_TARGET_TYPE_NONE - to mark that no type was
specified in the XML. This type is at the end converted to the standard
VIR_DOMAIN_CHR_CONSOLE_TARGET_TYPE_SERIAL. Other types that are
different from this default have to be processed separately in the
device post parse callback.