tcg: reorganize tb_find_physical loop

Put some comments and improve code structure. This should help reading
the code.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
[Sergey Fedorov: provide commit message; bring back resetting of
tb_invalidated_flag]
Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
Reviewed-by: Richard Henderson  <rth@twiddle.net>
Signed-off-by: Richard Henderson <rth@twiddle.net>
