KVM: SVM: document KVM_MEM_ENCRYPT_OP, let userspace detect if SEV is available

Userspace has no way to query if SEV has been disabled with the
sev module parameter of kvm-amd.ko.  Actually it has one, but it
is a hack: do ioctl(KVM_MEM_ENCRYPT_OP, NULL) and check if it
returns EFAULT.  Make it a little nicer by returning zero for
SEV enabled and NULL argument, and while at it document the
ioctl arguments.

Cc: Brijesh Singh <brijesh.singh@amd.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>