hw/core/null-machine: Add the possibility to instantiate a CPU and RAM

Sometimes it is useful to have just a machine with CPU and RAM, without
any further hardware in it, e.g. if you just want to do some instruction
debugging for TCG with a remote GDB attached to QEMU, or run some embedded
code with the "-semihosting" QEMU parameter. qemu-system-m68k already
features a "dummy" machine, and xtensa a "sim" machine for exactly this
purpose.
All target architectures have nowadays also a "none" machine, which would
be a perfect match for this, too - but it currently does not allow to add
CPU and RAM yet. Thus let's add these possibilities in a generic way to the
"none" machine, too, so that we hopefully do not need additional "dummy"
machines in the future anymore (and maybe can also get rid of the already
existing "dummy"/"sim" machines one day).
Note that the default behaviour of the "none" machine is not changed, i.e.
no CPU and no RAM is instantiated by default. You have explicitely got to
specify the CPU model with "-cpu" and the amount of RAM with "-m" to get
these new features.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-Id: <1484743490-24721-1-git-send-email-thuth@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@xilinx.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>