KVM: selftests: Move x86's descriptor table helpers "up" in processor.c

Move x86's various descriptor table helpers in processor.c up above
kvm_arch_vm_post_create() and vcpu_setup() so that the helpers can be
made static and invoked from the aforementioned functions.

No functional change intended.

Reviewed-by: Ackerley Tng <ackerleytng@google.com>
Link: https://lore.kernel.org/r/20240314232637.2538648-9-seanjc@google.com


Signed-off-by: Sean Christopherson <seanjc@google.com>