libxl: trigger attach events for devices attached before xl devd startup

libxl: trigger attach events for devices attached before xl devd startup

When this daemon is started after creating backend device, that device
will not be configured.

Racy situation:
1. driver domain is started
2. frontend domain is started (just after kicking driver domain off)
3. device in frontend domain is connected to the backend (as specified
   in frontend domain configuration)
4. xl devd is started in driver domain

End result is that backend device in driver domain is not configured
(like network interface is not enabled), so the device doesn't work.

Fix this by artifically triggering events for devices already present in
xenstore before xl devd is started. Do this only after xenstore watch is
already registered, and only for devices not already initialized (in
XenbusStateInitWait state).

Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>