xen/arm64: entry: Check if an SError is pending when receiving a vSError

xen/arm64: entry: Check if an SError is pending when receiving a vSError

At the moment, when we receive an SError exception from the guest, we
don't check if there are any other pending. For hardening the code, we
should ensure any pending SError are accounted to the guest before
executing any code with SError unmasked.

The recently introduced macro 'guest_vector' could used to generate the
two vectors and therefore take advantage of any change required in the
future.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Juergen Gross <jgross@suse.com>