qemuProcessReconnectHelper: Don't create joinable thread

qemuProcessReconnectHelper: Don't create joinable thread

In the qemuProcessReconnectHelper() a new thread that does all the
interesting work is spawned. The rationale is to not block the daemon
startup process in case of unresponsive qemu. However, the thread
handler is a local variable which gets lost once the control goes out of
scope. Hence the thread gets leaked. We can avoid this if the thread
isn't made joinable.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>