KVM: arm64: timers: Abstract the number of valid timers per vcpu

We so far have a pretty fixed number of timers to take care of.
This is about to change as NV brings another two into the
picture, and we must be careful not to try and emulate non-valid
timers in a given VM.

For this, abstract the number of timers for a given vcpu behind
an accessor, which helpfully returns a constant for now.

Reviewed-by: Colton Lewis <coltonlewis@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230330174800.2677007-15-maz@kernel.org