Reuse the XML files in storagevolxml2xmlin.
(This requires changing a few backing files to /dev/null,
since virStorageBackendCreateQemuImgCmd checks for its
presence)
Add <features> and <compat> elements to volume target XML.
<compat> is a string which for qcow2 represents the QEMU version
it should be compatible with. Valid values are 0.10 and 1.1.
1.1 is implicit if the <features> element is present, otherwise
qemu-img default is used. 0.10 can be specified to explicitly
create older images after the qemu-img default changes.
<features> contains optional features, so far
<lazy_refcounts/> is available, which enables caching of reference
counters, improving performance for snapshots.
