libxl/remus: introduce libxl__remus_setup

libxl/remus: introduce libxl__remus_setup

Refactoring Remus setup by introducing libxl__remus_setup API.
All Remus setup work are done in this function.

There is a subtle behavioural change here, which is that if anything
which is now done in _setup fails then the result is a call to
dss->callback( ..,..,ERROR_FAIL) rather than _start returning
AO_CREATE_FAIL(ERROR_FAIL). But this is correct and intended.

Also remove the libxl__ prefix for static functions.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>
CC: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>