Properly update iv_name of disks while changing templates

Trasforming the disk of an instance from DRBD to plain did not properly
update the iv_name of disks, leaving it to None.

This commit modifies _ConvertDrbdToPlain to properly set the iv_name
variables.

An analogous fix is not required in the opposite function,
_ConvertPlainToDrbd, because it uses _GenerateDiskTemplate, that already
takes care of doing that.

Fixes Issue 403.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>