tools/libxencall: use hypercall buffer device if available

tools/libxencall: use hypercall buffer device if available

Instead of using anonymous memory for hypercall buffers which is then
locked into memory, use the hypercall buffer device of the Linux
privcmd driver if available.

This has the advantage of needing just a single mmap() for allocating
the buffer and page migration or compaction can't make the buffer
unaccessible for the hypervisor.

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>