[libvirt] [PATCH] Only parse 'CPU XML' in virCPUDefParseXML()

I received a report of user crashing libvirtd with virsh capabilities > capabilities.xml virsh cpu-compare capabilities.xml Breakpoint 1, x86ModelFind (map=0x6fb6f0, name=0x0) at cpu/cpu_x86.c:501 501 model = map->models; (gdb) bt #0 x86ModelFind (map=0x6fb6f0, name=0x0) at cpu/cpu_x86.c:501 #1 0x00007ffff70d0832 in x86ModelFromCPU (cpu=0x712070, map=0x6fb6f0, policy=1) at cpu/cpu_x86.c:541 #2 0x00007ffff70d12e9 in x86Compute (host=0x6ea610, cpu=0x712070, guest=0x0) at cpu/cpu_x86.c:822 #3 0x00007ffff70d1750 in x86Compare (host=0x6ea610, cpu=0x712070) at cpu/cpu_x86.c:927 #4 0x00007ffff70cf0c9 in cpuCompare (conn=0x6f69a0, host=0x6ea610, cpu=0x712070) at cpu/cpu.c:118 #5 0x00007ffff70ceff4 in cpuCompareXML (conn=0x6f69a0, host=0x6ea610, xml=0x6fa3e0 "<capabilities>\n\n <host>\n <cpu>\n <arch>x86_64</arch>\n <features>\n <pae/>\n </features>\n </cpu>\n <migration_features>\n <live/>\n <uri_transports>\n <uri_tra"...) at cpu/cpu.c:90 .. (gdb) n 503 if (STREQ(model->name, name)) (gdb) Program received signal SIGSEGV, Segmentation fault. 0x00007ffff4ff4960 in strcmp () from /lib64/libc.so.6 name, passed to x86ModelFind(), is NULL because virCPUDefParseXML() failed to parse anything meaningful from the "malformed" CPU XML. I waffled for quite a while on how to fix this and in the end decided to ensure the CPU XML passed to virCPUDefParseXML() does in fact contain an element named 'cpu'. AFAICT, all consumers of virCPUDefParseXML() pass an xmlNodePtr rooted at 'cpu' element. NB: I will be offline until Monday so if this patch is satisfactory please include it for 0.7.8/0.8.0 release :-). Otherwise I'll respin with suggested changes next week. Thanks! Jim