CA-110160: Fixed uncaught exception in VM migrate wizard observed while concurrent migrate operation is triggered on same VM

- before leaving each page of the wizard, check if all selected VMs are still available for migration

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>
CA-115430: Fixed fatal exception while accessing VM context 'Migrate to Server' menu

- create a copy of the DropDownItems collection and use this copy to iterate through items (on the background thread)

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>
Merge branch 'master' of git://hg.uk.xensource.com/carbon/trunk/xenadmin
CA-121750: Added more checks to prevent potential NullReferenceException.
Merge commit '1504b184c78df141de91d732daa38bdc8a1a8636' into car-100