Check effective suspension of TCG thread

On multi-core systems, SuspendThread does not guaranty immediate thread
suspension. We add busy loop to wait for effective thread suspension
after call to ThreadSuspend().

Signed-off-by: Fabien Chouteau <chouteau@adacore.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
