[IA64] add VGCF_online flag for XEN_DOMCTL_setvcpucontext to make vcpu online.

[IA64] add VGCF_online flag for XEN_DOMCTL_setvcpucontext to make vcpu online.

Extend the XEN_DOMCTL_setvcpucontext hypercall with VGCF_online
to make vcpu online.
This extension is necessary for save/restore support of pv_ops linux guest.
Currently Linux kernel makes vcpus offline except a boot processor
(vcpu = 0) when saving so that only vcpu = 0 is made online by the tools
stack when restoring. Then Linux kernel itself makes other vcpus up.
On the other hand, pv_ops linux guest doesn't make vcpus offline.
It uses stop_machine_run() keeping all vcpus online.
Thus all of vcpus context must be saved/restored and all vcpus needs to be
made online by the tools stack. So there needs a way to make vcpu online.
Add VGCF_online flag to the XEN_DOMCTL_setvcpucontext hypercall
following the x86 way.

Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>