scsi-cd: check ready condition before processing several commands

This commit is more or less obvious.  What it caused is less obvious:
SCSI CD drives failed to eject under Linux, though for example the
"change" command worked okay.  This happens because of the autoclose
option in the Linux CD-ROM driver.

The actual chain of events is quite complex and somehow involves
udev helpers; the actual command that matters is READ TOC, though
honestly it's not really clear to me how because it should always be
invoked after autoclose, not before.

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