Add "ubd" to the list of disk prefixes

virDiskNameToIndex has a list of disk name prefixes that it uses in the
process of finding the disk's index. This list is missing "ubd" which
is the disk prefix used for UML domains.
Signed-off-by: NSoren Hansen <soren@linux2go.dk>

.mailmap

@@ -13,3 +13,4 @@
 <jfehlig@novell.com> <jfehlig@linux-ypgk.site>
 <jclift@redhat.com> <justin@salasaga.org>
 <berrange@redhat.com> <dan@berrange.com>
+<soren@canonical.com> <soren@linux2go.dk>

src/util/util.c

@@ -2367,7 +2367,7 @@ const char *virEnumToString(const char *const*types,
 int virDiskNameToIndex(const char *name) {
     const char *ptr = NULL;
     int idx = 0;
-    static char const* const drive_prefix[] = {"fd", "hd", "vd", "sd", "xvd"};
+    static char const* const drive_prefix[] = {"fd", "hd", "vd", "sd", "xvd", "ubd"};
     unsigned int i;
 
     for (i = 0; i < ARRAY_CARDINALITY(drive_prefix); i++) {