kdump pv-on-hvm: reset PV devices in crash kernel

kdump pv-on-hvm: reset PV devices in crash kernel

After triggering a crash dump in a HVM guest, the PV backend drivers
will remain in connected state.  When the kdump kernel starts the PV
drivers will skip such devices.  As a result, no root device is found
and the vmcore cant be saved.

With this change all frontend devices with state XenbusStateConnected
will be reset by changing the state file to
Closing/Closed/Initializing.  This will trigger a disconnect in the
backend drivers. Now the frontend drivers will find the backend
drivers in state Initwait and can connect.

Signed-off-by: Olaf Hering <olaf@aepfle.de>