Implement KVM_IA64_VCPU_[GS]ET_STACK ioctl calls. This is required
for live migrations.

Patch is based on previous implementation that was part of old
GET/SET_REGS ioctl calls.
Signed-off-by: Jes Sorensen <jes@sgi.com>
Signed-off-by: Avi Kivity <avi@redhat.com>