SCTX-1177: Preserve VM Settings (Optimized for Citrix XenApp)

When a VM is optimized for Citrix XenApp, the field HVM_shadow_multiplier = 4.

When this VM is shutdown, the field HVM_shadow_multiplier is reverted back to its default value which it obtains from the hatled_vm record. 

This patch prevents the xapi DB from updating the field HVM_shadow_multiplier on shutdown.