storage: fix memory leak with encrypted images
    
    Jim Fehlig reported a regression found by libvirt-TCK tests:
    
    > ~ # perl /usr/share/libvirt-tck/tests/qemu/100-disk-encryption.t
    ...
    > ok 4 - defined persistent domain config
    > # Starting inactive domain config
    > libvirt error code: 1, message: internal error: unable to execute QEMU command
    > 'cont': 'drive-ide0-0-1'
    > (/var/cache/libvirt-tck/300-disk-encryption/demo.qcow2) is encrypted
    
    Commit 2279d560 converted a boolean into a pointer with the intent of
    transferring that pointer out of a temporary object into the caller's
    data structure.  The temporary structure meant that meta->encryption
    was always NULL on entry, so we could get away with blindly allocating
    the pointer when the header said so.  But later, commit 8823272d
    tweaked things to do backing chain detection in-place, rather than via
    a temporary object; this has the net result that meta->encryption can
    be non-NULL on entry.  Not only did this turn the latent behavior into
    a memory leak, it is also a behavior regression: blindly allocating a
    new pointer wipes out what secrets we already knew about the chain,
    making it impossible to restart the domain.
    
    Of course, no one in their right mind should be relying on qcow2
    encryption - it is fundamentally flawed.  And sadly, the TCK tests
    don't get run often enough, and this shows that our virstoragetest
    does not exercise encrypted images at all.  Otherwise, we could
    have avoided a release containing this regression.
    
    * src/util/virstoragefile.c (virStorageFileGetMetadataInternal):
    Don't nuke an already-existing encryption.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>