QEMU has had two different syntax for disk cache options

 Old: on|off
 New: writeback|writethrough|none

QEMU recently added another 'unsafe' option which broke the
libvirt check. We can avoid this & future breakage, if we
do a negative check for the old syntax, instead of a positive
check for the new syntax

* src/qemu/qemu_conf.c: Invert cache option check