Instead of misusing a const string to hold up runtime allocated
data, introduce new variable @hoststr and obey const correctness.

==6879== 15 bytes in 1 blocks are definitely lost in loss record 68 of 1,064
==6879==    at 0x4C29F80: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==6879==    by 0xA7DDF97: vasprintf (in /lib64/libc-2.21.so)
==6879==    by 0x552BBC6: virVasprintfInternal (virstring.c:493)
==6879==    by 0x552BCDB: virAsprintfInternal (virstring.c:514)
==6879==    by 0x54FA44C: virLogHostnameString (virlog.c:468)
==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: NMichal Privoznik <mprivozn@redhat.com>