conf: Fix memory leak in graphics XML parser

conf: Fix memory leak in graphics XML parser

When loading status XMLs with following graphics definition:

  <graphics type='spice' port='5900' autoport='yes' listen='127.0.0.1'>
    <listen type='address' address='127.0.0.1' fromConfig='1'/>
    <image compression='off'/>
  </graphics>

libvirtd would leak a few bytes:

10 bytes in 1 blocks are definitely lost in loss record 71 of 1,127
   at 0x4C2C000: malloc (vg_replace_malloc.c:299)
   by 0x6789298: xmlStrndup (in /usr/lib64/libxml2.so.2.9.4)
   by 0x552AB0A: virXMLPropString (virxml.c:479)
   by 0x5539536: virDomainGraphicsListensParseXML (domain_conf.c:11171)
   by 0x553DD5E: virDomainGraphicsDefParseXMLSpice (domain_conf.c:11414)
   by 0x553DD5E: virDomainGraphicsDefParseXML (domain_conf.c:11749)
   by 0x5566061: virDomainDefParseXML (domain_conf.c:16939)
   by 0x556953F: virDomainObjParseXML (domain_conf.c:17348)
   by 0x556953F: virDomainObjParseNode (domain_conf.c:17513)
   by 0x5569902: virDomainObjParseFile (domain_conf.c:17532)
   by 0x5571E02: virDomainObjListLoadStatus (virdomainobjlist.c:514)
   by 0x5571E02: virDomainObjListLoadAllConfigs (virdomainobjlist.c:596)
   by 0x26E0BDC8: qemuStateInitialize (qemu_driver.c:911)
   by 0x55B1FDB: virStateInitialize (libvirt.c:770)
   by 0x122039: daemonRunStateInit (libvirtd.c:960)