testimage.bbclass: request the use of kvm by default
In almost all cases there is no need to explicitly ask for it via local.conf
or similar custom config:
it is enabled in qemu_use_kvm() if target_arch == build_arch or both of them are x86 archs,
and QEMU_USE_KVM is set.
If that heuristic doesn't work, QEMU_USE_KVM = "" will do the disabling.
Signed-off-by: Alexander Kanavin <alex@linutronix.de> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
