accel: use atomic accesses for exit_request

accel: use atomic accesses for exit_request

CPU threads write exit_request as a "note to self" that they need to
go out to a slow path.  This write happens out of the BQL and can be
a data race with another threads' cpu_exit(); use atomic accesses
consistently.

While at it, change the source argument from int ("1") to bool ("true").

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>