CA-110585: Passing correct snapshot_time during VDI.snapshot

Issue:for_vdi calls find_vdi which in turn refers to xapi db to find out
about vdi. At that point, the xapi db is not updated with the vdi info.
This leads to snapshot time being set to Date.never (1970-01-01). This
then gets passed to SM.

Fix: Adding an optional parameter vdi_info. This, if present, takes info
from vdi_info directly and doesnt peek xapi db for snapshot time.

Signed-off-by: Ravi Pandey <ravi.pandey@citrix.com>