machine: query mem-merge machine property

Running
    qemu-bin ... -machine pc,mem-merge=on
leads to crash:
    x86_64-softmmu/qemu-system-x86_64 -machine pc,dump-guest-core=on
    qemu-system-x86_64: qemu/util/qemu-option.c:387: qemu_opt_get_bool_helper:
    Assertion `opt->desc && opt->desc->type == QEMU_OPT_BOOL' failed.  Aborted
    (core dumped)

This happens because the commit e79d5a6 ("machine: remove qemu_machine_opts
global list") removed the global option descriptions and moved them to
MachineState's QOM properties.

Fix this by querying machine properties through designated wrappers.

Signed-off-by: Marcel Apfelbaum <marcel@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
