migration: protect the quit flag by lock

quit_comp_thread and quit_decomp_thread are accessed by several
thread, it's better to protect them with locks. We use a per
thread flag to replace the global one, and the new flag is protected
by a lock.

Signed-off-by: Liang Li <liang.z.li@intel.com>
Message-Id: <1462433579-13691-7-git-send-email-liang.z.li@intel.com>
Signed-off-by: Amit Shah <amit.shah@redhat.com>
