KVM: arm64: Handle out-of-bound write to MDCR_EL2.HPMN

We don't really pay attention to what gets written to MDCR_EL2.HPMN,
and funky guests could play ugly games on us.

Restrict what gets written there, and limit the number of counters
to what the PMU is allowed to have.

Reviewed-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>