Rework guest network configuration for QEMU+KVM demo

Rework guest network configuration for QEMU+KVM demo

Changes:
- Tweak agl-qemu-runner to make network options configurable.
- Add qemu-config-vmnet0 recipe that installs the required
  configuration files to set up a vmnet0 bridge device for QEMU to
  add the guests to, and to run dnsmasq against it to provide set
  IPs (in 172.16.10.0/8) to known MAC addresses that can be used to
  get the IVI and IC guests at stable IP addresses.
- As part of the above, disable connman DNS proxying to avoid
  conflicting with dnsmasq.  This does mean this package should
  only be used for simple host images where applications can live
  with less dynamic DNS configuration behavior.
- Update the various guest configuration files for agl-qemu-runner
  to set the new QEMU_NET_OPT variable as required.

Bug-AGL: SPEC-4618

Change-Id: I6807f08f11fc2f3aa4a5411b319ebc6d5283cdf8
Signed-off-by: Scott Murray <scott.murray@konsulko.com>