CA-139733: Stop vhd false alarm

Stop complaining the primary footer is invalid during storage motion
since the backup one will be read. Warning messages when reading backup
one is kept.

Signed-off-by: Kaifeng Zhu <kaifeng.zhu@citrix.com>
Reviewed-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>

GitHub: closes #118 on xapi-project/blktap