qemu_migrate: Dispose listen address if set from config

qemu_migrate: Dispose listen address if set from config

https://bugzilla.redhat.com/show_bug.cgi?id=971485

As of d7f9d827531bc843b7c5aa9d3e8c08738a1de248 we copy the listen
address from the qemu.conf config file in case none has been provided
via XML. But later, when migrating, we should not include such listen
address in the migratable XML as it is something autogenerated, not
requested by user. Moreover, the binding to the listen address will
likely fail, unless the address is '0.0.0.0' or its IPv6 equivalent.
This patch introduces a new boolean attribute to virDomainGraphicsListenDef
to distinguish autofilled listen addresses. However, we must keep the
attribute over libvirtd restarts, so it must be kept within status XML.