image_types: ensure .rootfs.ext3 is created before vmdk is created.

image_types: ensure .rootfs.ext3 is created before vmdk is created.

In the case of self-hosted-image.bb, IMAGE_FSTYPES = "vmdk", so the
variables alltypes and subimages don't contain ext3, and  .rootfs.ext3
won't be created, and finally the generated .hddimg and .vmdk don't have
an actual rootfs -- the size of the .vmdk file is only about 9MB.

[YOCTO #2067]

(From OE-Core rev: 39f14c66d5df226689238810b759f502644deb5c)

Signed-off-by: Dexuan Cui <dexuan.cui@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>