virDomainHostdevSubsysSCSIVHostDefParseXML: Don't leak @protocol

virDomainHostdevSubsysSCSIVHostDefParseXML: Don't leak @protocol

==24748== 12 bytes in 2 blocks are definitely lost in loss record 25 of 84
==24748==    at 0x4C2BF80: malloc (vg_replace_malloc.c:296)
==24748==    by 0x1A1E1E78: xmlStrndup (in /usr/lib64/libxml2.so.2.9.4)
==24748==    by 0x18D0495F: virXMLPropString (virxml.c:506)
==24748==    by 0x18D1FB3E: virDomainHostdevSubsysSCSIVHostDefParseXML (domain_conf.c:6280)
==24748==    by 0x18D20350: virDomainHostdevDefParseXMLSubsys (domain_conf.c:6450)
==24748==    by 0x18D34E7D: virDomainHostdevDefParseXML (domain_conf.c:13218)
==24748==    by 0x18D42598: virDomainDefParseXML (domain_conf.c:17745)
==24748==    by 0x18D440A9: virDomainDefParseNode (domain_conf.c:18236)
==24748==    by 0x18D43EFA: virDomainDefParse (domain_conf.c:18180)
==24748==    by 0x18D43FA0: virDomainDefParseFile (domain_conf.c:18206)
==24748==    by 0x44EDA1: testCompareDomXML2XMLFiles (testutils.c:1140)
==24748==    by 0x4365F8: testXML2XMLActive (qemuxml2xmltest.c:59)

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>