libxl: don't set gnttab limits in soft reset case

libxl: don't set gnttab limits in soft reset case

In case of soft reset the gnttab limit setting will fail, so omit it.
Setting of max vcpu count is pointless in this case, too, so we can
drop that as well.

Without this patch soft reset will fail with:

libxl: error: libxl_dom.c:363:libxl__build_pre: Couldn't set grant table limits

Reported-by: Jim Fehlig <jfehlig@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Tested-by: Jim Fehlig <jfehlig@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>