Ensure disk names follow the disk name regex

Currently disk names do not follow the
(regex) /^[fhv]d[a-z]+[0-9]*$/ completely
and hence one can assign disk names like
vd2 etc. This patch ensures that the
disk names follow the regex mentioned.
This patch also adds a testcase.
Signed-off-by: NNitesh Konkar <nitkon12@linux.vnet.ibm.com>

src/util/virutil.c

@@ -563,7 +563,7 @@ int virDiskNameParse(const char *name, int *disk, int *partition)
         }
     }
 
-    if (!ptr)
+    if (!ptr || !c_islower(*ptr))
         return -1;
 
     for (i = 0; *ptr; i++) {

tests/utiltest.c

@@ -37,7 +37,8 @@ static struct testDiskName diskNamesPart[] = {
 };
 
 static const char* diskNamesInvalid[] = {
-    "sda00", "sda01", "sdb-1"
+    "sda00", "sda01", "sdb-1",
+    "vd2"
 };
 
 static int