tcg: Pass last_tb by value to tb_find_fast()

tcg: Pass last_tb by value to tb_find_fast()

This is a small clean up. tb_find_fast() is a final consumer of this
variable so no need to pass it by reference. 'last_tb' is always updated
by subsequent cpu_loop_exec_tb() in cpu_exec().

This change also simplifies calling cpu_exec_nocache() in
cpu_handle_exception().

Signed-off-by: Sergey Fedorov <[email protected]>
Signed-off-by: Sergey Fedorov <[email protected]>
Message-Id: <20160715175852[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>