xen/arm: Save HCR_EL2 when a guest took the SError

xen/arm: Save HCR_EL2 when a guest took the SError

The HCR_EL2.VSE (HCR.VA for aarch32) bit can be used to generate a
virtual abort to guest. The HCR_EL2.VSE bit has a peculiar feature
of getting cleared when the guest has taken the abort (this is the
only bit that behaves as such in HCR_EL2 register).

This means that if we set the HCR_EL2.VSE bit to signal such an abort,
we must preserve it in the guest context until it disappears from
HCR_EL2, and at which point it must be cleared from the context. This
is achieved by reading back from HCR_EL2 until the guest takes the
fault.

If we preserved a pending VSE in guest context, we have to restore
it to HCR_EL2 when context switch to this guest. This is achieved
by writing saved HCR_EL2 value in guest context back to HCR_EL2
register before return to guest. This had been done by the patch
of "Restore HCR_EL2 register".

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