KVM: arm: Add support for early vcpu configuration ioctls

SVE will require the KVM_ARM_SVE_CONFIG ioctl to be used early to
configure a vcpu before other arch vcpu ioctls will behave in a
consistent way.

To hide these effects from userspace while minimising mess in the
generic code, this patch splits arch vcpu ioctls into two phases:
early configuration ioctls, and normal ioctls.  A new arch helper
vcpu_needs_configuration() reports whether any arch vcpu ioctls
other than early configuration ioctls are allowed.

This entire behaviour will be need to be opt-in for userspace.

There are currently no early configuration ioctls; one will be
added for SVE in a subsequent patch, along with a suitable opt-in
mechanism.

It is assumed that no core vcpu ioctls are affected by any of the
early configuration we want to do.  In any case, most "generic"
vcpu ioctls are stubbed out for arm/arm64 and return -EINVAL.

Signed-off-by: Dave Martin <Dave.Martin@arm.com>