pc: Require xen when initializing xenfv machine

Without this check, the xen-platform device will crash on reset
if using the accel option with anything other than xen (e.g.
"-machine xenfv,accel=kvm").

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>