target-i386: Rename x86_def_t to X86CPUDefinition

As the new X86CPU subclass code is going to change lots of the code
invoving x86_def_t, let's rename the struct to match coding style first.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>