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: NEric Blake <eblake@redhat.com>

tools/virsh-domain.c

@@ -2169,7 +2169,8 @@ cmdBlockCopy(vshControl *ctl, const vshCmd *cmd)
             if (bandwidth) {
                 /* bandwidth is ulong MiB/s, but the typed parameter is
                  * ullong bytes/s; make sure we don't overflow */
-                if (bandwidth + 0ULL > ULLONG_MAX >> 20) {
+                unsigned long long limit = MIN(ULONG_MAX, ULLONG_MAX >> 20);
+                if (bandwidth > limit) {
                     virReportError(VIR_ERR_OVERFLOW,
                                    _("bandwidth must be less than %llu"),
                                    ULLONG_MAX >> 20);