tools/libxendevicemodel: extract functions and add a compat layer

tools/libxendevicemodel: extract functions and add a compat layer

This patch extracts all functions resulting in a dm_op hypercall from
libxenctrl and moves them into libxendevicemodel. It also adds a compat
layer into libxenctrl, which can be selected by defining
XC_WANT_COMPAT_DEVICEMODEL_API to 1 before including xenctrl.h.

With this patch the core of libxendevicemodel still uses libxencall to
issue the dm_op hypercalls, but this is done by calling through code that
can be modified on a per-OS basis. A subsequent patch will add a Linux-
specific variant.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>