libxl: honor autoballoon setting in libxl.conf

libxl: honor autoballoon setting in libxl.conf

libxlGetAutoballoonConf is supposed to honor user-specified
autoballoon setting in libxl.conf. As written, the user-specified
setting could be overwritten by the subsequent logic to check
dom0_mem parameter. If user-specified setting is present and
correct, accept it. Only fallback to checking Xen dom0_mem
command line parameter if user-specfied setting is not present.

Signed-off-by: Jim Fehlig <jfehlig@suse.com>