nwfilter: Process DHCP option to determine whether packet is a DHCP_OFFER

nwfilter: Process DHCP option to determine whether packet is a DHCP_OFFER

I mistakenly took the op field in the DHCP message as the DHCP_OFFER
type. Rather than basing the decision to read the VM's IP address on
that field, process the appended DHCP options where option 53 indicates
the actual type of the packet. I am also reading the broadcast address
of the VM, but don't use it so far.