coroutine: move pooling to common code

coroutine: move pooling to common code

The coroutine pool code is duplicated between the ucontext and
sigaltstack backends, and absent from the win32 backend.  But the
code can be shared easily by moving it to qemu-coroutine.c.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>