If a file turns up with extension .vhd, set virtual_size to be the vhd's current_size

Previously we set the virtual_size to be the physical size of the .vhd
file on disk, which is obviously wrong. This manifests as a failing VDI.copy
as the target VDI is too small to contain the data.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Release 0.9.23

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #22 from djs55/get_virtual_size

SR.scan: fix the virtual_size for .vhds