xen/netfront: use iflladdr_event to send ARPs

Avoids usage of arp_ifinit() and if_foreach_addr_type(). The former
isn't encouraged to be used in drivers and the latter is about to
change to not expose struct ifaddr.

Reviewed by: royger, ehem_freebsd_m5p.com
Differential Revision: https://reviews.freebsd.org/D48053