Add support for running the harness with QEMU.

If ARCH makes it down as something other than 'um', the harness switches
to using QEMU instead of UML. It uses DEFCONFIG as a basis for the
kernel, but the networking flags are enabled/disabled in the same way.

Bug: 79694505
Test: Built tests locally, with and without UML
Change-Id: Ifbd638adceb4f148fd9c5aa0c37c5df63fe2505e
Signed-off-by: Alistair Strachan <astrachan@google.com>