tests: Store P2P Device ifname in class WpaSupplicant

tests: Store P2P Device ifname in class WpaSupplicant

Add an attribute to class WpaSupplicant with the name of the
P2P Device interface. If a separate interface is not used for
P2P Device, this attribute will hold the name of the only used
interface (with functions also as the P2P Device management
interface).

This attribute will be used to direct P2P related commands to the
P2P Device interface, which is needed for configurations that use
a separate interface for the P2P Device.

Signed-off-by: Avraham Stern <avraham.stern@intel.com>