libxl: wait for state "connected" in libxl__device_disk_local_attach

libxl: wait for state "connected" in libxl__device_disk_local_attach

In order to make sure that the block device is available and ready to be
used, wait for state "connected" in the backend before returning.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
[ ijc -- resolved simple conflict in libxl__device_disk_local_detach ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>