VM import/export now uses vhd-tool for disk streaming

In both cases we use less dom0 CPU and we use O_DIRECT.
A localhost export of a fresh 24GiB (5GiB used) win7
went from 4m48 to 1m:49, while import went from 4m:13
to 3m:34.

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