xen/pvh*: Support > 32 VCPUs at domain restore

When Xen restores a PVHVM or PVH guest, its shared_info only holds
up to 32 CPUs. The hypercall VCPUOP_register_vcpu_info allows
us to setup per-page areas for VCPUs. This means we can boot
PVH* guests with more than 32 VCPUs. During restore the per-cpu
structure is allocated freshly by the hypervisor (vcpu_info_mfn is
set to INVALID_MFN) so that the newly restored guest can make a
VCPUOP_register_vcpu_info hypercall.

However, we end up triggering this condition in Xen:
/* Run this command on yourself or on other offline VCPUS. */
 if ( (v != current) && !test_bit(_VPF_down, &v->pause_flags) )

which means we are unable to setup the per-cpu VCPU structures
for running VCPUS. The Linux PV code paths makes this work by
iterating over cpu_possible in xen_vcpu_restore() with:

 1) is target CPU up (VCPUOP_is_up hypercall?)
 2) if yes, then VCPUOP_down to pause it
 3) VCPUOP_register_vcpu_info
 4) if it was down, then VCPUOP_up to bring it back up

With Xen commit 192df6f9122d ("xen/x86: allow HVM guests to use
hypercalls to bring up vCPUs") this is available for non-PV guests.
As such first check if VCPUOP_is_up is actually possible before
trying this dance.

As most of this dance code is done already in xen_vcpu_restore()
let's make it callable on PV, PVH and PVHVM.

Based-on-patch-by: Konrad Wilk <konrad.wilk@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>