machine: replace qemu opts with iommu property

Fixes a QEMU crash when passing iommu parameter in command line.
Running
    x86_64-softmmu/qemu-system-x86_64 -machine pc,iommu=on -enable-kvm
leads to crash:
    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 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>
