[XEN] Add hypercall support for HVM guests. This is

[XEN] Add hypercall support for HVM guests.  This is
fairly useless at the moment, since all of the hypercalls
fail, since copy_from_user doesn't work correctly in HVM
domains.

Signed-off-by: Steven Smith <ssmith@xensource.com>
Add a CPUID hypervisor platform interface at leaf
0x40000000. Allow hypercall transfer page to be filled
in via MSR 0x40000000.

Signed-off-by: Keir Fraser <keir@xensource.com>