kvm: Ensure negative return value on kvm_init() error handling path

We need to ensure ret < 0 when going through the error path, or QEMU may
try to run the half-initialized VM and crash.

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