caps: Add new capability for the bps/iops throttling length

Add the capability to detect if the qemu binary can support the feature
to use bps-max-length and friends.

tests/qemucapabilitiesdata/caps_2.6.0-gicv2.aarch64.xml

@@ -158,6 +158,7 @@
   <flag name='display'/>
   <flag name='smm'/>
   <flag name='virtio-pci-disable-legacy'/>
+  <flag name='drive-iotune-max-length'/>
   <version>2005094</version>
   <kvmVersion>0</kvmVersion>
   <package></package>