vhost: split structs into a separate header file

vhost structs are shared by vhost-kernel and vhost-user.  Split them
into a separate file to ease copying them into programs that implement
either the server or the client side of vhost-user.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>