xen-netfront: improve the logic when handling nic features from ioctl

Simplify the logic involved in changing the nic features on the fly, and
only reset the frontend when really needed (when changing RX features). Also
don't return from the ioctl until the interface has been properly
reconfigured.

While there, make sure XN_CSUM_FEATURES is used consistently.

Reported by: julian
MFC after: 5 days
X-MFC-with: r303488
Sponsored by: Citrix Systems R&D