block/mirror: fix broken sparseness detection

int64_t is in all likelihood the actual scalar type we want.
Yep, really.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1219541

Signed-off-by: John Snow <jsnow@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Jeff Cody <jcody@redhat.com>
