qemuMigrationPrecreateStorage: Fix debug message
    
    When pre-creating storage for domains, we need to find corresponding
    disk in the XML on the destination (domain XML may differ there, e.g.
    disk is accessible under different path). For better debugging, I'm
    printing all info I received on a disk. But there was a typo when
    printing the disk capacity: "%lluu" instead of "%llu".
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>