domain_conf: fix possible memory leak

domain_conf: fix possible memory leak

Until now, it was possible to crash libvirtd when defining domain with
channel device with missing source element.

When creating new virDomainChrDef, target.port is set to -1, but
unfortunately it is an union with addresses that virDomainChrDefFree
tries to free in case the deviceType is channel. Having the port set
to -1 is intended, however the cleanest way to get around the problems
with the crash seems to be renumbering the VIR_DOMAIN_CHR_CHANNEL_
target types to cover new NONE type (with value 0) being the default
(no target type yet).