xen/arm: Introduce a virtual abort injection helper

xen/arm: Introduce a virtual abort injection helper

When guest triggers async aborts, in most platform, such aborts
will be routed to hypervisor. But we don't want the hypervisor
to handle such aborts, so we have to route such aborts back to
the guest.

This helper is using the HCR_EL2.VSE (HCR.VA for aarch32) bit to
route such aborts back to the guest. After updating HCR_EL2.VSE bit
in vCPU context, we write the value to HCR_EL2 immediately. In this
case we don't need to move the restoration of HCR_EL2 to other place,
and it worked regardless of whether we get preempted.

If the guest PC had been advanced by SVC/HVC/SMC instructions before
we caught the SError in hypervisor, we have to adjust the guest PC to
exact address while the SError generated.

About HSR_EC_SVC32/64, even thought we don't trap SVC32/64 today,
we would like them to be handled here. This would be useful when
VM introspection will gain support of SVC32/64 trapping.

After updating HCR_EL2.VSE bit of vCPU HCR_EL2, write the value
to HCR_EL2 immediately. In this case we don't need to move the
restoration of HCR_EL2 to leave_hypervisor_tail, and it worked
regardless of whether we get preempted.

This helper will be used by the later patches in this series, we
use #if 0 to disable it in this patch temporarily to remove the
warning message of unused function from compiler.

Signed-off-by: Wei Chen <Wei.Chen@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>