Improve interrupt handling priority

The vector interrupt has higher priority than interrupt_level_n.
Also check only interrupt_level_n concurency when TL > 0, the traps of
other types may be nested.

Signed-off-by: Artyom Tarasenko <atar4qemu@gmail.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
