target-i386: forward CPUID cache leaves when -cpu host is used

Some users running cpu intensive tasks checking the cache CPUID leaves at
startup and making decisions based on the result reported that the guest was
not reflecting the host CPUID leaves when -cpu host is used.

This patch fix this.

Signed-off-by: Benoît Canet <benoit@irqsave.net>
[Rename new field to cache_info_passthrough - Paolo]
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
