xen/netback: do not attempt to connect in the Initialised state

Only attempt to fetch the configuration data and connect the shared
ring once the frontend has switched to the 'Connected' state. This
seems to be inline with what Linux netback does, and is required to
make newer versions of NetBSD netfront work, since NetBSD only
publishes the required configuration before switching to the Connected
state.

Sponsored by: Citrix Systems R&D
(cherry picked from commit 4489124c04727a4aad418eec3148e0c8de23ff4d)