conf: fix crash when parsing a unordered NUMA <cell/>

conf: fix crash when parsing a unordered NUMA <cell/>

https://bugzilla.redhat.com/show_bug.cgi?id=1260846

Introduced by 8fedbbdb, if we parse an unordered NUMA cell, will
get a segfault. This is because of a check for overlapping @cpus
sets we have there. However, since the array to hold guest NUMA
cells is allocated upfront and therefore it contains all zeros,
an out of order cell will break our assumption that cell IDs have
increasing character. At this point we try to access yet NULL
bitmap and therefore segfault.

Signed-off-by: Luyao Huang <lhuang@redhat.com>