domain_conf: Introduce chardev hotplug helpers

domain_conf: Introduce chardev hotplug helpers

For now, only these three helpers are needed:
virDomainChrFind - to find a duplicate chardev within VM def
virDomainChrInsert - wrapper for inserting a new chardev into VM def
virDomainChrRemove - wrapper for removing chardev from VM def

There is, however, one internal helper as well:
virDomainChrGetDomainPtrs which sets given pointers to one of
vmdef->{parallels,serials,consoles,channels} based on passed
chardev type.