xen-netfront: perform an interface reset when changing options

[royger]

Summary

The PV backend will only pick the new options when the interface is detached
and reattached again, so perform a full reset when changing options. This is
very fast, and should not be noticeable by the user.

Sponsored by: Citrix Systems R&D

[liuw_liuw.name]

Can't comment on the feature flags to freebsd flags mapping.

Overall this reconnection approach looks sensible to me. The good thing is that is doesn't require any additional extension for the protocol so we can always switch to something better in the future,