util: Refactor thread creation by introducing virThreadPoolExpand

util: Refactor thread creation by introducing virThreadPoolExpand

When either creating a threadpool, or creating a new thread to accomplish a job
that had been placed into the jobqueue, every time thread-specific data need to
be allocated, threadpool needs to be (re)-allocated and thread count indicators
updated. Make the code clearer to read by compressing these operations into a
more complex one.

Signed-off-by: Erik Skultety <eskultet@redhat.com>