cpu: make cpu_generic_init() abort QEMU on error

Almost every user of cpu_generic_init() checks for
returned NULL and then reports failure in a custom way
and aborts process.
Some users assume that call can't fail and don't check
for failure, though they should have checked for it.

In either cases cpu_generic_init() failure is fatal,
so instead of checking for failure and reporting
it various ways, make cpu_generic_init() report
errors in consistent way and terminate QEMU on failure.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <1505318697-77161-3-git-send-email-imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>