nm: device: refactor create_index_for_nm_devs

Use &str instead of String to avoid cloning all the interface names and
store the NmDevices as refs. This index is short lived so it's OK to
restrict its lifetime to the original list of devices.

Use InterfaceType instead of NmIfaceType in the index so the caller
doesn't need to do the conversion.

This function will be used in the next commit.

Signed-off-by: Íñigo Huguet <ihuguet@riseup.net>