ci: QEMU_USE_KVM needs empty variable

QEMU_USE_KVM needs to be empty, not 0.  Otherwise, it doesn't catch and
runs KVM anyway (which breaks inside our docker containers).

Signed-off-by: Jon Mason <jon.mason@arm.com>

ci/base.yml

@@ -38,7 +38,7 @@ local_conf_header:
   noptest: |
     DISTRO_FEATURES:remove = "ptest"
   kvm: |
-    QEMU_USE_KVM = "0"
+    QEMU_USE_KVM = ""
 
 machine: unset