tools/libxl: Prepare to write multiple records with EMULATOR headers

tools/libxl: Prepare to write multiple records with EMULATOR headers

With the newly specified EMULATOR_XENSTORE_DATA record, there are two
libxl records with an emulator subheader.  Refactor the existing code to
make future additions easier, and rename some functions for consistency
with the new scheme.

* Calculate the subheader at stream start time, rather than on the fly.
  Its contents are not going to change.
* Introduce a new setup_emulator_write() to insert a sub header in the
  appropriate place before a blob of data.
* Rename *toolstack_* to *emulator_xenstore_*
* Rename *emulator_* to *emulator_context_*

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <Ian.Campbell@citrix.com>