kvm tools: Enable kernel DHCP autoconf by default

As we boot to '/bin/sh' by default, enable kernel DHCP autoconfiguration too.
The guest kernel needs to be configured with the following configuration
options:

  CONFIG_IP_PNP=y
  CONFIG_IP_PNP_DHCP=y

Cc: Asias He <asias.hejun@gmail.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Prasad Joshi <prasadjoshi124@gmail.com>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>