Remove flush_operations in the volume usage output

flush_operations is not used by upper layer code and also
according to
ftp://libvirt.org/libvirt/java/javadoc/org/libvirt/DomainBlockStats.html
http://libvirt.org/ruby/api/Libvirt/Domain/BlockStats.html

seems the last value should be errs instead of flush_operations
so remove it from libvirt driver layer.

Change-Id: I438b6fa40e9d97e3ffddda03919c5d577e91c155
Merge "Remove flush_operations in the volume usage output"