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: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>