KVM: SVM: Return -EINVAL instead of -EBUSY on attempt to re-init SEV/SEV-ES

Return -EINVAL instead of -EBUSY if userspace attempts KVM_SEV{,ES}_INIT
on a VM that already has SEV active.  Returning -EBUSY is nonsencial as
it's impossible to deactivate SEV without destroying the VM, i.e. the VM
isn't "busy" in any sane sense of the word, and the odds of any userspace
wanting exactly -EBUSY on a userspace bug are minuscule.

Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20240131235609.4161407-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>