KVM: arm64: Drop redundant READ_ONCE() in pkvm_hyp_vm_is_created()

is_created is written under config_lock. Every concurrent reader is
serialised against that write: pkvm_create_hyp_vm() under config_lock,
and the memslot path (kvm_arch_prepare_memory_region) via slots_lock,
which the creation writer also holds. The teardown-path accesses have no
concurrent writer. The read is therefore serialised, and the READ_ONCE()
is unnecessary.

Signed-off-by: Fuad Tabba <tabba@google.com>