block/vdi: allow disk sizes not multiple of block size

The disk image I created from my old laptop disk with VBoxManage
internalcommand converthd obviously was not a multiple of 1MB as when
created from scratch. This fixes QEMU refusing it. We still require the
size to be a multiple of sector size though.
It then boots correctly.

Allow opening VDI images with size not multiple of 1MB (as when converted from a raw disk).

Signed-off-by: François Revol <revol@free.fr>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>