net: add missing "netmap" to host_net_devices[]

Although hmp-commands.hx lists "netmap" as a valid host_net_add type,
the command rejects it because it's missing from the list.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1432743412-15943-2-git-send-email-stefanha@redhat.com