blockjob: avoid 32-bit compilation warning
    
    Commit c1d75de caused this warning on 32-bit platforms (fatal when
    -Werror is enabled):
    
    virsh-domain.c: In function 'cmdBlockCopy':
    virsh-domain.c:2003:17: error: comparison is always false due to limited range of data type [-Werror=type-limits]
    
    Forcing the left side of the < to be ull instead of ul shuts up
    the 32-bit compiler while still protecting 64-bit code from overflow.
    
    * tools/virsh-domain.c (cmdBlockCopy): Add type coercion.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>