x86: allow HVM guests to use hypercalls to bring up vCPUs

x86: allow HVM guests to use hypercalls to bring up vCPUs

Allow the usage of the VCPUOP_initialise, VCPUOP_up, VCPUOP_down,
VCPUOP_is_up, VCPUOP_get_physid and VCPUOP_send_nmi hypercalls from HVM
guests.

This patch introduces a new structure (vcpu_hvm_context) that should be used
in conjuction with the VCPUOP_initialise hypercall in order to initialize
vCPUs for HVM guests.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>