xenlight: make VBD.plug idempotent

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
xenlight: give read-only VBDs to Xenlight.domain_create_restore

This ensures that those VBDs are plugged before QEMU is started, which seems to
be necessary for CD-ROMs to work properly.

Signed-off-by: Rob Hoes <rob.hoes@citrix.com>
Merge pull request #39 from robhoes/restore

xenlight: give read-only VBDs to Xenlight.domain_create_restore