qemu-img is going to switch the default for QCOW2
to QCOW2v3 (compat=1.1)

Extend the probing for qemu-img command line options to check
if -o compat is supported. If the volume definition specifies
the qcow2 format but no compat level and -o compat is supported,
specify -o compat=0.10 to create a QCOW2v2 image.

https://bugzilla.redhat.com/show_bug.cgi?id=997977

Ignore NULL pool in testSetVolumeType to silence Coverity,
even though we only call it with NULL pool when vol is also NULL.

(13) Event var_deref_model:	Passing null pointer "inputpool" to
function "testSetVolumeType(virStorageVolDefPtr, virStoragePoolDefPtr)",
which dereferences it. [details]
Also see events: [assign_zero]

95    testSetVolumeType(inputvol, inputpool);

This allows testing the command line for cloning file-based
volumes into logical volumes and vice versa.

Reuse the XML files in storagevolxml2xmlin.

(This requires changing a few backing files to /dev/null,
since virStorageBackendCreateQemuImgCmd checks for its
presence)

Reuse the pool definition from storagepoolxml2xmlin.

Reindent them to put the input volume on a separate line.

Actually, I'm turning this function into a macro as filename,
function name and line number needs to be passed. The new
function virAsprintfInternal is introduced with the extended set
of arguments.

Add -o compat= and -o lazy_refcounts options for qemu-img.

If the creation of the commandline failed, libvirt always reported "out
of memory" from the virCommandToString function rather than the proper
error that happened in virStorageBackendCreateQemuImgCmd. Error out
earlier.

The source code base needs to be adapted as well. Some files
include virutil.h just for the string related functions (here,
the include is substituted to match the new file), some include
virutil.h without any need (here, the include is removed), and
some require both.

Reported by John Ferlan.