virtual-sensor: Add ability to read a config from dbus

If we match on the right interface we attempt to add a virtual sensor
from a configuration on dbus.

As we do not want to take arbitrary expressions from dbus, we only match
on a pre-determined set of calculations that can be an exprtk expression
or a function. One is added in a later commit.

Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I228f60fa0f484cd1e7be1aca3d097494c5168936
2 files changed