CA-107600: While performing VM_appliance.shutdown first attempt to perform VM_appliance.clean_shutdown and if that fails we then fallback to VM_appliance.hard_shutdown. 

Issue: VM_appliance.shutdown was unable to shutdown suspended VMs in a vApp. 

Solution:
This patch first tries to clean_shutdown all the VMs in the appliance. 
For those VMs in the vApp that couldn't be couldn't be clean_shutdown, we fallback to hard_shutdown. 

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>