Move global variables into a struct so multiple thread pools can be
supported in the future.

This patch does not change thread-pool.h interfaces.  There is still a
global thread pool and it is not yet possible to create/destroy
individual thread pools.  Moving the variables into a struct first makes
later patches easier to review.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NPaolo Bonzini <pbonzini@redhat.com>