CA-97232: Optimize host-evacuate
When host-evacuate is in progress:
Case-1: Ignore VMs that are being shutdown
Case-2: Prevent VMs from starting on that host

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Adding error message to datamodel

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Adding additional check in Xapi_vm_helpers.vm_can_run_on_host for VM.Start

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Merge pull request #1113 from siddharthv/CA-97232-C12

CA-97232: Optimize host-evacuate