cpu: Fix crash with empty -cpu option

Fix the following crash:

  $ qemu-system-x86_64 -cpu ''
  qemu-system-x86_64: qom/cpu.c:291: cpu_class_by_name: \
      Assertion `cpu_model && cc->class_by_name' failed.

Regression test script included.

Fixes: 99193d8f ("cpu: drop unnecessary NULL check and cpu_common_class_by_name()")
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20190418034501.5038-1-ehabkost@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Tested-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>