target-ppc: explicitly save page table headers in big endian

Currently, when the page tables are saved, the kvm_get_htab_header structs
and the ptes are assumed being big endian and dumped as a indistinct blob
in the statefile. This is no longer true when the host is little endian
and it breaks restoration.

This patch unfolds the kvmppc_save_htab routine to write explicitly the
kvm_get_htab_header structs in big endian. The ptes are left untouched.

Signed-off-by: Cédric Le Goater <clg@fr.ibm.com>