power: Make test_power driver more dynamic.

In 2008 Masashi YOKOTA <yokota@pylone.jp> created the virtual
battery driver found here:
http://downloads.pylone.jp/src/virtual_battery/virtual_battery-0.0.1.tar.bz2

It found use out of tree, but was never merged upstream.
Since then the test_power driver has been merged, which provides
very similar functionality.

This patch extends the test_power driver to be more dynamic
at runtime, by merging portions of the Virtual Battery Driver
by Masashi YOKOTA.

With this patch, I can tweak the values in:
/sys/module/test_power/parameters/* and watch the behavior of
the gnome power managment daemon or other battery UI software.

CC: Anton Vorontsov <cbouatmailru@gmail.com>
CC: Akihiro MAEDA <sola.1980.a@gmail.com>
CC: Masashi YOKOTA <yokota@pylone.jp>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>