kvm: selftests: introduce aarch64_vcpu_setup

This allows aarch64 tests to run on more targets, such as the Arm
simulator that doesn't like KVM_ARM_TARGET_GENERIC_V8. And it also
allows aarch64 tests to provide vcpu features in struct kvm_vcpu_init.
Additionally it drops the unused memslot parameters.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>