qemu: command: Support new cpu feature argument syntax

qemu: command: Support new cpu feature argument syntax

Qemu has abandoned the +/-feature syntax in favor of key=value. Some
architectures (s390) do not support +/-feature. So we update libvirt to handle
both formats.

If we detect a sufficiently new Qemu (indicated by support for qmp
query-cpu-model-expansion) we use key=value else we fall back to +/-feature.

Signed-off-by: Collin L. Walling <walling@linux.vnet.ibm.com>
Signed-off-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>