block: Fix backing paths for filenames with colons

path_combine() naturally tries to preserve a protocol prefix. However,
it recognizes such a prefix by scanning for the first colon; which is
different from what path_has_protocol() does: There only is a protocol
prefix if there is a colon before the first slash.

A protocol prefix that is not recognized by path_has_protocol() is none,
and should thus not be taken as one.

Case in point, before this patch:
$ ./qemu-img create -f qcow2 -b backing.qcow2 ./top:image.qcow2
qemu-img: ./top:image.qcow2: Could not open './top:backing.qcow2':
    No such file or directory

Afterwards:
$ ./qemu-img create -f qcow2 -b backing.qcow2 ./top:image.qcow2
qemu-img: ./top:image.qcow2: Could not open './backing.qcow2':
    No such file or directory
Reported-by: Nyangyang <yangyang@redhat.com>
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Message-id: 20170522195217.12991-2-mreitz@redhat.com
Signed-off-by: NMax Reitz <mreitz@redhat.com>

block.c

@@ -163,11 +163,16 @@ void path_combine(char *dest, int dest_size,
     if (path_is_absolute(filename)) {
         pstrcpy(dest, dest_size, filename);
     } else {
-        p = strchr(base_path, ':');
-        if (p)
-            p++;
-        else
-            p = base_path;
+        const char *protocol_stripped = NULL;
+
+        if (path_has_protocol(base_path)) {
+            protocol_stripped = strchr(base_path, ':');
+            if (protocol_stripped) {
+                protocol_stripped++;
+            }
+        }
+        p = protocol_stripped ?: base_path;
+
         p1 = strrchr(base_path, '/');
 #ifdef _WIN32
         {