scsi: clarify sense codes for LUN0 emulation

The LUN0 emulation is just that, an emulation for a non-existing
LUN0. So we should be returning LUN_NOT_SUPPORTED for any request
coming from any other LUN.
And we should be aborting unhandled commands with INVALID OPCODE,
not LUN NOT SUPPORTED.

Signed-off-by: Hannes Reinecke <hare@suse.com>
Message-Id: <1501835795-92331-4-git-send-email-hare@suse.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>