libxl: convert strings and ints to xenbus_state

libxl: convert strings and ints to xenbus_state

Convert all plain ints and strings which are used for xenbus "state"
files to xenbus_state. This makes it easier to find code which deals
with backend/frontend state changes.

Convert usage of libxl__sprintf to GCSPRINTF.

No change in behaviour is expected by this change, beside a small
increase of runtime memory usage in places that used a string constant.

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Wei Liu <wei.liu2@citrix.com>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>