blockjob: avoid 32-bit compilation warning

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>