xen-netfront: perform an interface reset when changing options

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.

Reviewed by: Wei Liu <wei.liu2@citrix.com>
Sponsored by: Citrix Systems R&D
Differential revision: https://reviews.freebsd.org/D6658