BAT: round up when computing the total number of blocks

We round up when calculating the BAT size, so we cover all
codepaths. Also add a sanity check in the tests that all
vhds have an apparent capacity (indicated by the BAT size)
sufficient to record the full vhd current_size.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
stream: add the ability to stream a raw disk as vhd

Signed-off-by: David Scott <dave.scott@eu.citrix.com>