x86/boot: Wire up dom0=shadow for PV dom0

x86/boot: Wire up dom0=shadow for PV dom0

This would have been very handy when debugging some pv-l1tf issues.  As there
is no cost to supporting it, wire it up.

Due to the way dom0 is constructed, switching into shadow mode must be done
after the pagetables are written, and because of partially being in dom0
context, shadow_enable() doesn't like the state it finds.

Reuse the pv_l1tf tasklet for convenience, which will switch dom0 into shadow
mode just before it starts executing.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>