block/vpc: Fix conversion from size to disk geometry

The VHD algorithm calculates a disk geometry
which is usually smaller than the requested size.

QEMU tried to round up but failed for certain sizes:

qemu-img create -f vpc disk.vpc 9437184
would create an image with 9435136 bytes
(which is too small for qemu-img convert).

Instead of hacking the geometry algorithm, the patch
increases the number of sectors until we get enough
sectors.

Cc: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
