virLogHostnameString: Don't leak hostname
    
    Once @hostname is printed into @hoststr we don't need it anymore.
    
    ==6879== 5 bytes in 1 blocks are definitely lost in loss record 10 of 1,064
    ==6879==    at 0x4C29F80: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==6879==    by 0xA7ED599: strdup (in /lib64/libc-2.21.so)
    ==6879==    by 0x552C126: virStrdup (virstring.c:726)
    ==6879==    by 0x553B13E: virGetHostnameImpl (virutil.c:720)
    ==6879==    by 0x553B1BF: virGetHostnameQuiet (virutil.c:741)
    ==6879==    by 0x54FA3FD: virLogHostnameString (virlog.c:462)
    ==6879==    by 0x54FAB0F: virLogVMessage (virlog.c:645)
    ==6879==    by 0x54FA680: virLogMessage (virlog.c:531)
    ==6879==    by 0x54FBBF4: virLogParseOutputs (virlog.c:1130)
    ==6879==    by 0x11CB4F: daemonSetupLogging (libvirtd.c:685)
    ==6879==    by 0x11E137: main (libvirtd.c:1297)
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>