atapi: Add 'medium ready' to 'medium not ready' transition on cd change

MMC-5 Table F.1 lists errors that can be thrown for the TEST_UNIT_READY
command.  Going from medium not ready to medium ready states is
communicated by throwing an error.

This adds the missing 'tray opened' event that we fail to report to
guests.  After doing this, older Linux guests properly revalidate a disc
on the change command.  HSM violation errors, which caused Linux guests
to do a soft-reset of the link, also go away:

ata2.00: exception Emask 0x0 SAct 0x0 SErr 0x0 action 0x6
sr 1:0:0:0: CDB: Test Unit Ready: 00 00 00 00 00 00
ata2.00: cmd a0/00:00:00:00:00/00:00:00:00:00/a0 tag 0
         res 01/60:00:00:00:00/00:00:00:00:00/a0 Emask 0x3 (HSM violation)
ata2.00: status: { ERR }
ata2: soft resetting link
ata2.00: configured for MWDMA2
ata2: EH complete

Signed-off-by: Amit Shah <amit.shah@redhat.com>
Acked-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Tested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
