libxl: make libxl_cd_insert "eject" + "insert"

libxl: make libxl_cd_insert "eject" + "insert"

We introduce an intermediate empty state when inserting media into
CDROM. The scheme works like this:

  lock json config
  write empty state to xenstore
  for (;;) {
      write user supplied disk to JSON
      write disk information to xenstore
  }
  unlock json config

Bear in mind that all steps can fail. With the proposed scheme, we now
know, if xenstore is empty, then CDROM should be considered empty;
otherwise we should use JSON version of CDROM configuration.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>