s390x: replace cpu_s390x_init() with cpu_generic_init()

cpu_s390x_init() is used only *-user targets indirectly
via cpu_init() macro and has a hack to assign ids to created
cpus (I'm not sure if 'id' really matters to *-user emulation).

So to on safe side, instead of having custom wrapper to do numbering
replace it with cpu_generic_init() and use S390CPUClass::next_cpu_id
which could serve the same purpose as static variable and move cpu->id
initialization to s390_cpu_initfn for CONFIG_USER_ONLY use-case.

PS:
ifdef is ugly but it allows us to hide s390x detail that isn't
set by *-user targets and reuse generic cpu creation utility
for btoh machine and user emulation.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Message-Id: <1504185578-80843-1-git-send-email-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
