vpp-swan: fix function to get sw_if_index

vpp-swan: fix function to get sw_if_index

In the current implementation there is a bug in the function
responsible for getting software interface index by the name of the
interface. Incorrect function is used to send the API message, also
the handler with replied message is incorrect.

The fix changes function to send dump message and also adds handler
with replied message in the correct way.

Type: fix
Signed-off-by: Gabriel Oginski <gabrielx.oginski@intel.com>
Change-Id: Id1a3ba2ce7e92d216907f344431b9e2acb1d5572