xen/arm: Prevent slipping hypervisor SError to guest

xen/arm: Prevent slipping hypervisor SError to guest

If there is a pending SError while we're returning from trap. If the
SError handle option is "DIVERSE", we have to prevent slipping this
hypervisor SError to guest. So we have to use the dsb/isb to guarantee
that the pending hypervisor SError would be caught in hypervisor before
return to guest.

In previous patch, we will set SKIP_SYNCHRONIZE_SERROR_ENTRY_EXIT to
cpu_hwcaps when option is NOT "DIVERSE". This means we can use the
alternative to skip synchronizing SErrors for other SErrors handle options.

Because we have umasked the Abort/SError bit in previous patch. We have
to disable the Abort/SError before returning to guest as we have done
for IRQ.

Signed-off-by: Wei Chen <Wei.Chen@arm.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>