loader.cc: don't start DHCP when no interface is found

KVM always have a network interface provided by QEMU, Xen not necessarily so. I
fixed this before, but it was probably broken by the multiple interfaces work.
The interface list is not a C++ structure, so the easiest way to handle that is
to keep track of whether or not we have any non-loopback interface. (No need to
know how many). If we don't, skip DHCP startup.

Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>