qemu-options: Rename variables on the -numa "cpus" option

Use @var{firstcpu} and @var{lastcpu} to make the metasyntatic
variables a bit clearer. While doing this, use @var only around
the metasyntatic variables, not including the square brackets and
hyphen.

The semantics of the "cpus" option will be clarified by rewriting
the whole -numa documentation in a follow-up patch.

Reported-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170123180632.28942-2-ehabkost@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>