Refactor varadic argument log formating

Handling of varadic log formating. It used to be duplicated
in 4 functions.

Create a new vLog function, taking a va_list as parameter
and formating the log. All 4 previous mention varadic argument
log functions now forward their argument to vLog.

Signed-off-by: Kevin Rocard <kevinx.rocard@intel.com>
vasprintf fails by returning -1

int vasprintf(char **strp, const char *fmt, va_list ap)
return -1 on failure, not by setting strp to NULL.

Replace check for NULL by checking for -1 return.

Signed-off-by: Kevin Rocard <kevinx.rocard@intel.com>