cpu: Change default for CPUClass::get_paging_enabled()

cpu: Change default for CPUClass::get_paging_enabled()

qemu_get_guest_memory_mapping() uses cpu_paging_enabled() to determine
whether to use cpu_get_memory_mapping() to return mappings or whether to
fall back to a simple identity map.

Since by default CPUClass::get_memory_mapping() is not implemented,
change the default to false to use the identity map by default.

Reviewed-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Reviewed-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>