xen: Drop __XEN_LATEST_INTERFACE_VERSION__ checks from prior to Xen 4.2

We assume (and check for in configure) 4.2 or later now. In reality
all of the removed checks are for far older versions.

FMT_ioreq_size is no longer needed.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
