KVM: arm64: Share the parts of get/set events useful to 32bit

The get/set events helpers to do some work to check reserved
and padding fields are zero. This is useful on 32bit too.

Move this code into virt/kvm/arm/arm.c, and give the arch
code some underscores.

This is temporarily hidden behind __KVM_HAVE_VCPU_EVENTS until
32bit is wired up.

Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Dongjiu Geng <gengdongjiu@huawei.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>