tools/libxl: Save and restore EMULATOR_XENSTORE_DATA content

tools/libxl: Save and restore EMULATOR_XENSTORE_DATA content

The new EMULATOR_XENSTORE_DATA content is a sequence of NUL terminated
key/value strings, with the key relative to the device model's xenstore
tree.

A sample might look like (as decoded by verify-stream-v2):

    Emulator Xenstore Data (Qemu Upstream, idx 0)
      'physmap/1f00000/start_addr' = 'f0000000'
      'physmap/1f00000/size' = '800000'
      'physmap/1f00000/name' = 'vga.vram'

This patch introduces libxl helpers to save and restore this new format,
which reimplement the existing libxl__toolstack_{save,restore}() logic.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>