libxl: split libxl_domain_shutdown into libxl_domain_shutdown,..._reboot

libxl: split libxl_domain_shutdown into libxl_domain_shutdown,..._reboot

The other integer request types which shutdown supported are not
useful. Specifically:

 * "suspend" is not usable via this interface since it requires other
   scaffolding, libxl_domain_suspend provides this already.
 * "halt" is the same as "poweroff".
 * "crash" is unused and at least Linux does not implement it. If a user
   steps forward then libxl_domain_crash is trivial to add.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson.citrix.com>
Acked-by: Ian Jackson <ian.jackson.citrix.com>