VMDK3 header has the field l1dir_size, but vmdk_open_vmdk3 hardcoded the
value. This patch honors the header field.

And the L2 table size is 4096 according to VMDK spec[1], instead of
1 << 9 (512).

[1]:
http://www.vmware.com/support/developer/vddk/vmdk_50_technote.pdf?src=vmdk



Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>