target/s390x/cpu_models: Rework the output of "-cpu help"

target/s390x/cpu_models: Rework the output of "-cpu help"

Printing an "s390x" in front of each CPU name is not helpful at all:
It is confusing for the users since they don't know whether they
have to specify these letters for the "-cpu" parameter, too, and
it also takes some precious space in the dense output of the CPU
entries. Let's simply remove this now!

While we're at it, use two spaces at the beginning of the lines for
the indentation of the entries, and add a "Available CPUs" in the
very first line, like most other target architectures are doing it
for their "-cpu help" output already.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>