libxl: Introduce libxl_domain_create_new() and libxl_domain_create_restore()

libxl: Introduce libxl_domain_create_new() and libxl_domain_create_restore()

These functions are introduced as the new way to create domains with libxl
they prevent the callers from need to know about low-level implementation
details such as:
 - libxl_domain_make()
 - libxl_domain_build()
 - libxl_domain_restore()
 - when to attach the console
 - how to start the device model

Above mentioned functions and all API's for the device model, which are now
redundant, have been made internal to libxl and no longer accessible.

The ocaml binding for libxl has not been properly updated to reflect the
changes, wrappers for the old functions have been removed but the code to wrap
the new functions has not been added.

Signed-off-by: Gianni Tedesco <gianni.tedesco@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>