qemu-img: fix invalid JSON

Single quotes for JSON are a QMP-ism, use real JSON in
qemu-img output.

Reported-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/qemu-img.c b/qemu-img.c
index 6a1ba69..926f0a0 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -1839,7 +1839,7 @@
                (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('}');