Send unit_attention on cd-rom not_ready to ready transition

Send unit_attention on cd-rom not_ready to ready transition

Patch sends a UNIT_ATTENTION(6), MEDIUM_MAY_HAVE_CHANGED(0x28) sense
when cdrom transitions from not ready to ready.

I used the following documentation:
 ATA Packet interface for CD-ROMS, SFF8020i.pdf. See state diagram Figure
 12, page 82 and Table 44 -recommended Sense Key, ASC

With patch in place HVM win2008 server guest sees the CD/DVD contents
have changed when the media is switched.

Signed-off-by: Pat Campbell <plc@novell.com>