disk: Use virStorageBackendZeroPartitionTable

https://bugzilla.redhat.com/show_bug.cgi?id=1439132

During 'matrix' testing of all possible combinations I found that if
device is formated with "gpt" first, then an attempt is made to format
using "mac", a startup will fail.

Deeper analysis by Peter Krempa indicates that the "mac" table fits
into the first block on the disk. Since the GPT disklabel is stored
at LBA address 1 it is not overwritten at all. Thus it's apparent that
the (blkid) detection tool then prefers GPT over a older disklabel.

The GPT disklabel has also a secondary copy at the last LBA of the disk.

So, follow the same logic as the logical pool in clearing a 1MB swath
at the beginning and end of the device to avoid potential issues with
larger sector sizes for the device.

Also fixed a minor formatting nit in virStorageBackendDeviceIsEmpty call.

src/storage/storage_backend_disk.c

@@ -491,11 +491,15 @@ virStorageBackendDiskBuildPool(virConnectPtr conn ATTRIBUTE_UNUSED,
         ok_to_mklabel = true;
     } else {
         if (virStorageBackendDeviceIsEmpty(pool->def->source.devices[0].path,
-                                              fmt, true))
+                                           fmt, true))
             ok_to_mklabel = true;
     }
 
     if (ok_to_mklabel) {
+        if (virStorageBackendZeroPartitionTable(pool->def->source.devices[0].path,
+                                                1024 * 1024) < 0)
+            goto error;
+
         /* eg parted /dev/sda mklabel --script msdos */
         if (format == VIR_STORAGE_POOL_DISK_UNKNOWN)
             format = pool->def->source.format = VIR_STORAGE_POOL_DISK_DOS;