[libvirt] dnsmasq SRV/TXT RR and Host xml parsing upgrade and bug fix

The set of patches aims to three kind of jobs. First, for SRV RR, in order to strictly comform to RFC 2782, the patch changed and upgraded codes in following parts. The format of SRV RR: _Service._Proto.Name TTL Class SRV Priority Weight Port Target 1,The protocol and service labels are prepended with an underscore. e.g: _ldap._tcp.example.com 2,The range of Port,Priority,Weight is 0-65535. 3,A Target of "." means that the service is decidedly not available at this domain. Also, trying to invoke dnsmasq command line based on its manpage. --srv-host=<_service>.<_prot>.[<domain>],[<target>[,<port>[,<priority>[,<weight>]]]] _service and protocol are mandatory options. The reset of them are optional. When the target is not given in dnsmasq, it comform to the third item above. For TXT RR, we changed the text field from mandatory to optional based on dnsmasq manpage: --txt-record=<name>[[,<text>],<text>] Second, changed the way of dns element as well as its children elements parsing via virXPathNodeSet(). It has advantages when multiple children elements there is. Third, fixed related bug and unexposed bugs https://bugzilla.redhat.com/show_bug.cgi?id=836326