xen: Fix scheduler setting problems

xen: Fix scheduler setting problems

Doing `virsh schedinfo rhel5u3 --cap 65535' the hypervisor does the
call, but does not change the value nor raise an error. Best is just to
consider it's not in the allowed values. The problem is that the error
won't be output since the xend driver will then be called and raise an
error

    error: this function is not supported by the hypervisor: unsupported
    in xendConfigVersion < 4

which will override the useful information from
xenUnifiedDomainSetSchedulerParameters(). So best is to also invert the
order in which the xen sub-drivers are called.

* src/xen/xen_hypervisor.c: mark 65535 cap value as out of bound
* src/xen/xen_hypervisor.c: reverse the order of the calls to the xen
  sub drivers to get the error message if needed