qemuBuildNumaArgStr: Simplify @nodeBackends

Instead of array of pointers to individual buffers it can be
array of buffers directly. This also fixes the following memleak:

==22516== 96 bytes in 4 blocks are definitely lost in loss record 166 of 195
==22516== at 0x4C2EF26: calloc (vg_replace_malloc.c:711)
==22516== by 0x5D2C7D5: virAlloc (viralloc.c:144)
==22516== by 0x56FAABD: qemuBuildNumaArgStr (qemu_command.c:7543)
==22516== by 0x5701835: qemuBuildCommandLine (qemu_command.c:10112)
==22516== by 0x575D794: qemuProcessCreatePretendCmd (qemu_process.c:6568)
==22516== by 0x113338: testCompareXMLToArgv (qemuxml2argvtest.c:549)
==22516== by 0x138CA3: virTestRun (testutils.c:180)
==22516== by 0x136CD1: mymain (qemuxml2argvtest.c:2825)
==22516== by 0x13AD58: virTestMain (testutils.c:1118)
==22516== by 0x137351: main (qemuxml2argvtest.c:2874)

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