build: avoid 32-bit failure on older gcc
    
    On 32-bit platforms with old gcc (hello RHEL 5 gcc 4.1.2), the
    build fails with:
    virsh-domain.c: In function 'cmdBlockCopy':
    virsh-domain.c:2172: warning: comparison is always false due to limited range of data type
    
    Adjust the code to silence the warning.
    
    * tools/virsh-domain.c (cmdBlockCopy): Pacify RHEL 5 gcc.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>