xen: Use stable library interfaces when they are available.

In Xen 4.7 we are refactoring parts libxenctrl into a number of
separate libraries which will provide backward and forward API and ABI
compatiblity.

Specifically libxenevtchn, libxengnttab and libxenforeignmemory.

Previous patches have already laid the groundwork for using these by
switching the existing compatibility shims to reflect the intefaces to
these libraries.

So all which remains is to update configure to detect the libraries
and enable their use. Although they are notionally independent we take
an all or nothing approach to the three libraries since they were
added at the same time.

The only non-obvious bit is that we now open a proper xenforeignmemory
handle for xen_fmem instead of reusing the xen_xc handle.

Build tested with 4.0 .. 4.6 (inclusive) and the patches targetting
4.7 which adds these libraries.

This uses CONFIG_XEN_CTRL_INTERFACE_VERSION == 471 to cover the
introduction of these new interfaces.

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>