KVM: PPC: Add kvm_book3s_32.h

In analogy to the 64 bit specific header file, this is the 32 bit
pendant. With this in place we can just always call to_svcpu and
be assured we get the right pointer anywhere.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Avi Kivity <avi@redhat.com>