vmdk: Fix creating big description file

The buffer for description file was 4096 which only covers a few
hundred of extents. This changes the buffer to dynamic allocated with
g_strdup_printf in order to support bigger cases.

Signed-off-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit af057fe7)

Conflicts:

	block/vmdk.c
	tests/qemu-iotests/059
	tests/qemu-iotests/059.out

*removed dependencies on 4823970b and 4f6fd349

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>