vhdx: Bounds checking for block_size and logical_sector_size (CVE-2014-0148)

Other variables (e.g. sectors_per_block) are calculated using these
variables, and if not range-checked illegal values could be obtained
causing infinite loops and other potential issues when calculating
BAT entries.

The 1.00 VHDX spec requires BlockSize to be min 1MB, max 256MB.
LogicalSectorSize is required to be either 512 or 4096 bytes.

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