CA-229331: evacuate: Don't report memory issues for unmigratable VMs.
CA-229331: evacuate: Don't report memory issues for unmigratable VMs.

Also, don't report more than one error per VM in Host.compute_evacuation_plan

Signed-off-by: Jon Ludlam <e7e3380887a8f95cc9dc4f0d51dedc7e849a287a@citrix.com>

Frezzle/xen-api,thomassa/xen-api,euanh/xen-api,Frezzle/xen-api,thomassa/xen-api,srowe/xen-api,srowe/xen-api,thomassa/xen-api,euanh/xen-api,thomassa/xen-api,euanh/xen-api,euanh/xen-api,Frezzle/xen-api,srowe/xen-api,srowe/xen-api,srowe/xen-api,Frezzle/xen-api,Frezzle/xen-api,euanh/xen-api,euanh/xen-api,thomassa/xen-api,thomassa/xen-api