xenapi: Fix VDI:read_only, VDI:sharable and VBD:mode of XenAPI

xenapi: Fix VDI:read_only, VDI:sharable and VBD:mode of XenAPI

I started a VM by using xm create, then I checked values of VDI
records and values of VBD records.  When I gave the following disk
modes to a disk parameter, I got the following values from the records.

               "r"    "w"    "w!"
VDI:read_only  True   True   True   <-- Always True!
VDI:sharable   True   True   True   <-- Always True!
VBD:mode       RO     RW     RO
                             ^^  <-- It should be RW.

This patch fixes the values of the records as follows.

               "r"    "w"    "w!"
VDI:read_only  True   False  False
VDI:sharable   False  False  True
VBD:mode       RO     RW     RW

Signed-off-by: Masaki Kanno <kanno.masaki@jp.fujitsu.com>