cli: add a 'hybrid' format along with 'vhd' and 'raw'

A 'hybrid' source is expected to take the form 'raw_source:vhd_source'
and the stream will contain copies from 'raw_source' based on BAT
information provided via 'vhd_source'.

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
vhd: Vhd_IO.get_sector_location returns a sector

For consistency this function works entirely in sector offsets.
It simply maps a virtual sector offset to a physical sector offset.

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