Fix virDiskNameToIndex to actually ignore partition numbers

The docs for virDiskNameToIndex claim it ignores partition
numbers. In actual fact though, a code ordering bug means
that a partition number will cause the code to accidentally
multiply the result by 26.
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/util/util.c

@@ -2193,11 +2193,10 @@ int virDiskNameToIndex(const char *name) {
         return -1;
 
     for (i = 0; *ptr; i++) {
-        idx = (idx + (i < 1 ? 0 : 1)) * 26;
-
         if (!c_islower(*ptr))
             break;
 
+        idx = (idx + (i < 1 ? 0 : 1)) * 26;
         idx += *ptr - 'a';
         ptr++;
     }