kvm tools: Handle DHCP package in gernal UDP processing path

DHCP packages are handled by the hypervisor, they are not sent to remote
servers. In contrast, all other non-DHCP UDP packages are sent to remote
servers.

Signed-off-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>