pxe_query_cpus: small fix of pxe_query_cpus

1. judge whether the thread is alive in every cpu info query.

2. add join() bg thread in the body.

3. set 'kill_vm_gracefully = no' in the cfg.

Signed-off-by: Cong Li <coli@redhat.com>
Merge pull request #86 from CongSmile/pxe-query-cpus

pxe_query_cpus: small fix of pxe_query_cpus