KVM: s390: move s390-specific structs to uapi/asm/kvm.h

While this in principle breaks the appearance of KVM_S390_* ioctls on architectures
other than s390, this seems unlikely to be a problem considering that there are
already many "struct kvm_s390_*" definitions in arch/s390/include/uapi.

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