libxl: Fix libxl_set_memory_target return value

libxl: Fix libxl_set_memory_target return value

libxl_set_memory_target seems to have the following return values:

 '1' : on failure, if the failure happens because of a xenstore error
       *or* invalid target
 '-1': on error, the setmaxmem and set_pod_target hypercalls
       return -1 and set errno appropriately.
 '0' : on success

Make it consistently return ERROR_FAIL on failure, unless the
parameters were invalid, in which case return ERROR_INVAL.

In accordance with CODING_SYTLE:

  1. Leave rc uninitialized, and set when an error is detected

  2. Use 'r' for return values to functions whose return values are a
     different error space (like xc_domain_setmaxmem and
     xc_domain_set_pod_target)

  3. Use 'lrc' for return values to local functions libxl__*
     where a failure means retry, rather than fail the whole function
     (libxl__fill_dom0_memory_info), to reduce the risk of that.

Signed-off-by: George Dunlap <George.Dunlap@eu.citrix.com>
Signed-off-by: Paulina Szubarczyk <paulinaszubarczyk@gmail.com>
Reviewed-by: Olaf Hering <olaf@aepfle.de>
Acked-by: Wei Liu <wei.liu2@citrix.com>