KVM: arm64: Keep a per-VM pointer to the default PMU

As we are about to allow selection of the PMU exposed to a guest, start by
keeping track of the default one instead of only the PMU version.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Link: https://lore.kernel.org/r/20220127161759.53553-4-alexandru.elisei@arm.com