Fix memory leak:

==27534== 24 bytes in 1 blocks are definitely lost in loss record 207 of 530
==27534==    at 0x4A05E46: malloc (vg_replace_malloc.c:195)
==27534==    by 0x38EC26EC37: vasprintf (in /lib64/libc-2.13.so)
==27534==    by 0x4E998E6: virVasprintf (util.c:1677)
==27534==    by 0x4E999F1: virAsprintf (util.c:1695)
==27534==    by 0x4F1EAAC: nodeGetInfo (nodeinfo.c:593)
==27534==    by 0x47948F: qemuCapsInitCPU (qemu_capabilities.c:855)
==27534==    by 0x4796B1: qemuCapsInit (qemu_capabilities.c:915)
==27534==    by 0x456550: qemuCreateCapabilities (qemu_driver.c:245)
==27534==    by 0x4578C4: qemudStartup (qemu_driver.c:580)
==27534==    by 0x4F20886: virStateInitialize (libvirt.c:852)
==27534==    by 0x420E55: daemonRunStateInit (libvirtd.c:1156)
==27534==    by 0x4E94C56: virThreadHelper (threads-pthread.c:157)

Mark this leaked variable as const char * when it is passed into another
function.