There is no point in accessing the HW when writing to any of the
ISENABLER/ICENABLER registers from userspace, as only the guest
should be allowed to change the HW state.

Introduce new userspace-specific accessors that deal solely with
the virtual state.
Reported-by: NJames Morse <james.morse@arm.com>
Tested-by: NJames Morse <james.morse@arm.com>
Reviewed-by: NJames Morse <james.morse@arm.com>
Signed-off-by: NMarc Zyngier <maz@kernel.org>