scsi-disk: parse MODE SELECT commands and parameters

This adds the bulk of the parsing code for MODE SELECT, including
breaking out changes to different mode pages, and checking that only
changeable values are modified.

In order to report errors correctly two passes are made through the
parameters; the first only looks for errors, the second actually
applies the changes to the mode page.

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