qdev: trigger reset from a given device

Introduce a helper function which triggers reset from a given device.
Will be used by pci bus emulation.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>