avoid newly-introduced test failure

* tests/qemuxml2argvdata/qemuxml2argv-hugepages.args: Update
expected output to match, now that we use -mem-prealloc.

tests/qemuxml2argvdata/qemuxml2argv-hugepages.args

-LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test /usr/bin/qemu -S -M pc -m 214 -mem-path /dev/hugepages/libvirt/qemu -smp 1 -nographic -monitor unix:/tmp/test-monitor,server,nowait -no-acpi -boot c -hda /dev/HostVG/QEMUGuest1 -net none -serial none -parallel none -usb
+LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test /usr/bin/qemu -S -M pc -m 214 -mem-prealloc -mem-path /dev/hugepages/libvirt/qemu -smp 1 -nographic -monitor unix:/tmp/test-monitor,server,nowait -no-acpi -boot c -hda /dev/HostVG/QEMUGuest1 -net none -serial none -parallel none -usb