vdi: add bounds checks for blocks_in_image and disk_size header fields (CVE-2014-0144)

The maximum blocks_in_image is 0xffffffff / 4, which also limits the
maximum disk_size for a VDI image to 1024TB.  Note that this is the maximum
size that QEMU will currently support with this driver, not necessarily the
maximum size allowed by the image format.

This also fixes an incorrect error message, a bug introduced by commit
5b7aa9b56d1bfc79916262f380c3fc7961becb50 (Reported by Stefan Weil)

Signed-off-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
