cpu-exec: fix cpu_exec_nocache

In icount mode cpu_exec_nocache function is used to execute part of the
existing TB. At the end of cpu_exec_nocache newly created TB is deleted.
Sometimes io_read function needs to recompile current TB and restart TB
lookup and execution. After that tb_find_fast function finds old (bigger)
TB again. This TB cannot be executed (because icount is not big enough)
and cpu_exec_nocache is called again. Such a loop continues over and over.
This patch deletes old TB and avoids finding it in the TB cache.

Signed-off-by: Pavel Dovgalyuk <Pavel.Dovgaluk@ispras.ru>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>