scsi: Handle no media case for scsi_get_configuration

Currently, scsi_get_configuration always returns a current
profile (DVD or CD), even when there is actually no media present.
By comparison, ide/atapi uses a default profile of 0 (MMC_PROFILE_NONE)
for this case and checks for tray_open, so let's do the same for scsi.

This fixes a problem I'm seeing with Fedora 22 guests where systemd
cdrom_id fails to unmount after a QEMU-initiated eject against a
scsi cdrom device because it believes the media is still present
(but unreadable).

Signed-off-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
Message-Id: <1436986352-10695-1-git-send-email-mjrosato@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
