kunit: tool: introduce --qemu_args

Example usage:
$ ./tools/testing/kunit/kunit.py run --arch=x86_64 \
  --kconfig_add=CONFIG_SMP=y --qemu_args='-smp 8'

Looking in the test.log, one can see
> smp: Bringing up secondary CPUs ...
> .... node  #0, CPUs:      #1 #2 #3 #4 #5 #6 #7
> smp: Brought up 1 node, 8 CPUs

This flag would allow people to make tweaks like this without having to
create custom qemu_config files.

For consistency with --kernel_args, we allow users to repeat this
argument, e.g. you can tack on a --qemu_args='-m 2048', or you could
just append it to the first string ('-smp 8 -m 2048').

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>