qtail: clean up direct access to tqe_prev field

instead of accessing tqe_prev field dircetly outside
of queue.h use macros to check if element is in list
and make sure that afer element is removed from list
tqe_prev field could be used to do the same check.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <1469450832-84343-1-git-send-email-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>