iface: Raise error when trying to merge ifaces with different type

For `gretap` interface, nispor 2.0.0 is incorrectly treating it as
`InterfaceType::Ethernet` while NetworkManager is treating it as
`InterfaceType::IpTunnel`, when nmstate merging these two interfaces, we
got wired error:

    Invalid YAML string: interfaces: unknown field `ethernet`

This is caused by `merge_desire_with_current()` copy `ethernet` section
to a `ip-tunnel` interface.

This patch changed `merge_desire_with_current()` to raise error instead
of blindly merging two interface literally.

Unit test case included.

Signed-off-by: Gris Ge <fge@redhat.com>