KVM: arm64: Add vm fd device attribute accessors

A subsequent change will allow userspace to convey a filter for
hypercalls through a vm device attribute. Add the requisite boilerplate
for vm attribute accessors.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230404154050.2270077-4-oliver.upton@linux.dev