qemu-img: fix invalid JSON

Single quotes for JSON are a QMP-ism, use real JSON in
qemu-img output.
Reported-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qemu-img.c

@@ -1839,7 +1839,7 @@ static void dump_map_entry(OutputFormat output_format, MapEntry *e,
                (e->flags & BDRV_BLOCK_ZERO) ? "true" : "false",
                (e->flags & BDRV_BLOCK_DATA) ? "true" : "false");
         if (e->flags & BDRV_BLOCK_OFFSET_VALID) {
-            printf(", 'offset': %"PRId64"", e->offset);
+            printf(", \"offset\": %"PRId64"", e->offset);
         }
         putchar('}');