xen: Use libxendevicemodel for emulated MSI handling

xen: Use libxendevicemodel for emulated MSI handling

Create the necessary shim layer for older versions of Xen, as usual
using the new library API as the basis.

We need to drop the configure check for xc_hvm_inject_msi from the
iface=471 case, since it will go away around the same time that
libxendevicemodel is introduced. Other factors in this block will
ensure we don't detect 471 in error.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>