blockcopy: add qemu implementation of new API
    
    The hard part of managing the disk copy is already coded; all
    this had to do was convert the XML and virTypedParameters into
    the internal representation.
    
    With this patch, all blockcopy operations that used the old
    API should also work via the new API.  Additional extensions,
    such as supporting the granularity tunable or a network rather
    than file destination, will be added as later patches.
    
    * src/qemu/qemu_driver.c (qemuDomainBlockCopy): New function.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>