replay: exception replay fix

This patch fixes replaying the exception when TB cache is full.
It breaks cpu loop execution through setting exception_index
to process such queued work as TB flush.

v8: moved setting of exeption_index to tb_gen_code

Signed-off-by: Pavel Dovgalyuk <pavel.dovgaluk@ispras.ru>
Message-Id: <20170126123418.5412.33815.stgit@PASHA-ISP>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
