Fix shrinking volumes with the delta flag
    
    This never worked.
    
    In 0.9.10 when this API was introduced, it was intended that
    the SHRINK flag combined with DELTA would shrink the volume by
    the specified capacity (to avoid passing negative numbers).
    See commit 055bbf4.
    
    When the SHRINK flag was finally implemented for the first backend
    in 1.2.13 (commit aa9aa6a), it was only implemented for the absolute
    values and with the delta flag the volume is always extended,
    regardless of the SHRINK flag.
    
    Treat the SHRINK flag as a minus sign when used together with DELTA,
    to allow shrinking volumes as was documented in the API since 0.9.10.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1220213