qemu: bulk stats: Ignore errors from missing/inaccessible disks

Rather than erroring out make the best attempt to retrieve other data if
disks are inaccessible or missing. The failure will still be logged
though.

Since the bulk stats API is called on multiple domains an error like
this makes the API unusable. This regression was introduced by commit
596a1371

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1209394

src/qemu/qemu_driver.c

@@ -19301,8 +19301,13 @@ qemuDomainGetStatsOneBlock(virQEMUDriverPtr driver,
             ret = 0;
             goto cleanup;
         }
-        if (qemuStorageLimitsRefresh(driver, cfg, dom, src) < 0)
+
+        if (qemuStorageLimitsRefresh(driver, cfg, dom, src) < 0) {
+            virResetLastError();
+            ret = 0;
             goto cleanup;
+        }
+
         if (src->allocation)
             QEMU_ADD_BLOCK_PARAM_ULL(record, maxparams, block_idx,
                                      "allocation", src->allocation);