vpc is not really a passthrough driver, even when using the fixed
subformat (where host and guest offsets are equal).  It should handle
preallocation like all other drivers do, namely by returning
DATA | RECURSE instead of RAW.

There is no tangible difference but the fact that bdrv_is_allocated() no
longer falls through to the protocol layer.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Message-id: 20190725155512.9827-4-mreitz@redhat.com
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>