tools/libxendevicemodel: introduce a Linux-specific implementation

tools/libxendevicemodel: introduce a Linux-specific implementation

My recent patch [1] to the Linux privcmd module introduced a dedicated
mechanism for making dm_op hypercalls.

This patch adds the necessary code to libxendevicemodel to take
advantage of that mechanism if it is implemented in the tools domain
kernel.

[1] https://git.kernel.org/cgit/linux/kernel/git/ostr/linux.git/commit/?id=ab520be8

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