Make check for /dev/loop device names stricter to avoid /dev/loop-control

Recentish (2011) kernels introduced a new device called /dev/loop-control,
which causes libvirt's detection of loop devices to get confused
since it only checks for a prefix of 'loop'. Also check that the
next character is a digit
Signed-off-by: NDaniel P. Berrange <berrange@redhat.com>

src/util/virfile.c

@@ -546,7 +546,11 @@ static int virFileLoopDeviceOpen(char **dev_name)
 
     errno = 0;
     while ((de = readdir(dh)) != NULL) {
-        if (!STRPREFIX(de->d_name, "loop"))
+        /* Checking 'loop' prefix is insufficient, since
+         * new kernels have a dev named 'loop-control'
+         */
+        if (!STRPREFIX(de->d_name, "loop") ||
+            !c_isdigit(de->d_name[4]))
             continue;
 
         if (virAsprintf(&looppath, "/dev/%s", de->d_name) < 0)