Allow shrinking of file based volumes
    
    While the main storage driver code allows the flag
    VIR_STORAGE_VOL_RESIZE_SHRINK to be set, none of the backend
    drivers are supporting it. At the very least this can work
    for plain file based volumes since we just ftruncate() them
    to the new size. It does not work with qcow2 volumes, but we
    can arguably delegate to qemu-img for error reporting for that
    instead of second guessing this for ourselves:
    
    $ virsh vol-resize --shrink /home/berrange/VirtualMachines/demo.qcow2 2G
    error: Failed to change size of volume 'demo.qcow2' to 2G
    
    error: internal error: Child process (/usr/bin/qemu-img resize /home/berrange/VirtualMachines/demo.qcow2 2147483648) unexpected exit status 1: qemu-img: qcow2 doesn't support shrinking images yet
    qemu-img: This image does not support resize
    
    See also https://bugzilla.redhat.com/show_bug.cgi?id=1021802