xen: make it possible to build without the Xen PV domain builder

xen: make it possible to build without the Xen PV domain builder

Until the previous patch this relied on xc_fd(), which was only
implemented for Xen 4.0 and earlier.

Given this wasn't working since Xen 4.0 I have marked this as disabled
by default.

Removing this support drops the use of a bunch of symbols from
libxenctrl, specifically:

  - xc_domain_create
  - xc_domain_destroy
  - xc_domain_getinfo
  - xc_domain_max_vcpus
  - xc_domain_setmaxmem
  - xc_domain_unpause
  - xc_evtchn_alloc_unbound
  - xc_linux_build

This is another step towards only using Xen libraries which provide a
stable inteface.

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>