qemu-thread: implement joinable threads for POSIX

qemu-thread: implement joinable threads for POSIX

Allow to control if a QEMU thread is created joinable or not. Make it
not joinable by default to avoid that we keep the associated resources
around when terminating a thread without joining it (what we couldn't do
so far for obvious reasons).

The audio subsystem will need the join feature when converting it to
QEMU threading/locking abstractions, so provide that service.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>