pv-qemu 6/10: Merge private & public xenfb structs

pv-qemu 6/10: Merge private & public xenfb structs

This patch merges the public & private structs from the paravirt FB
into a single struct. Since QEMU is the only consumer of this code
there is no need  for the artifical pub/priv split. Merging the two
will make it possible to more tightly integrate with QEMU's event
loop and do asynchronous non-blocking negoiation with the frontend
devices (see next patch).

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>