x86/pv: Move stack_switch()/seg_segment_base() into PV-only files
So they are excluded from !CONFIG_PV builds.
andrew.cooper3@citrix.com
Signed-off-by: Andrew CooperCC: Jan Beulich JBeulich@suse.com CC: Roger Pau Monné roger.pau@citrix.com CC: Wei Liu wl@xen.org