virsh: Allow using complete <capabilities> elements with cpu-baseline

virsh: Allow using complete <capabilities> elements with cpu-baseline

This patch cleans the cpu baseline function using new libvirt helper
functions and fixes XPath expression that selects <cpu> elements from
the source file, that can contain concatenated <capabilities> XMLs,
domain XMLs and bare <cpu> elements. The fixed XPath expression ensures
not to select NUMA <cpu id=... elements.

This patch also removes vshRealloc function, that remained unused after
cleaning up cpu-baseline.

https://bugzilla.redhat.com/show_bug.cgi?id=731645