KVM: arm64: Add is_pkvm_initialized() helper

Add a helper allowing to check when the pkvm static key is enabled to
ease the introduction of pkvm hooks in other parts of the code.

Signed-off-by: Quentin Perret <qperret@google.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Oliver Upton <oliver.upton@linux.dev>
Link: https://lore.kernel.org/r/20240423150538.2103045-18-tabba@google.com


Signed-off-by: Marc Zyngier <maz@kernel.org>