target/ppc: Move setting of patb_entry on hash table init

The patb_entry is used to store the location of the process table in
guest memory. The msb is also used to indicate the mmu mode of the
guest, that is patb_entry & 1 << 63 ? radix_mode : hash_mode.

Currently we set this to zero in spapr_setup_hpt_and_vrma() since if
this function gets called then we know we're hash. However some code
paths, such as setting up the hpt on incoming migration of a hash guest,
call spapr_reallocate_hpt() directly bypassing this higher level
function. Since we assume radix if the host is capable this results in
the msb in patb_entry being left set so in spapr_post_load() we call
kvmppc_configure_v3_mmu() and tell the host we're radix which as
expected means addresses cannot be translated once we actually run the cpu.

To fix this move the zeroing of patb_entry into spapr_reallocate_hpt().

Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
