Use macvtap0 interface for realm guest test case

macvtap along with FVP tap interface allows the host booted on FVP
and realm guests booted on host to reach outside world over network.

This will be required to run attestation demo where keybroker server
or verifier would be running outside the local network.

Also, remove ssh connection from host to guest test as it doesn't work
as libvirt on host where FVP is run doesn't provide any route and
blocks traffic.

Signed-off-by: Jagdish Gediya <jagdish.gediya@arm.com>