docs/qcow2: Limit refcount_order to [0, 6]

Specify the upper limit of refcount_order to be 6 (that is,
refcount_bits = 64). Any larger value does not make much sense when all
offsets, sizes, cluster counts etc. "only" have a width of 64 bit as
well, and very large values would be very difficult to support.
Therefore, just cap it at the largest reasonable value.
Suggested-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

docs/specs/qcow2.txt

@@ -110,6 +110,7 @@ in the description of a field.
                     in bits: refcount_bits = 1 << refcount_order). For version 2
                     images, the order is always assumed to be 4
                     (i.e. refcount_bits = 16).
+                    This value may not exceed 6 (i.e. refcount_bits = 64).
 
         100 - 103:  header_length
                     Length of the header structure in bytes. For version 2