target/ppc: Yet another fix for KVM-HV HPTE accessors

target/ppc: Yet another fix for KVM-HV HPTE accessors

As stated in the 1ad9f0a464fe commit log, the returned entries are not
a whole PTEG. It was not a problem before 1ad9f0a464fe as it would read
a single record assuming it contains a whole PTEG but now the code tries
reading the entire PTEG and "if ((n - i) < invalid)" produces negative
values which then are converted to size_t for memset() and that throws
seg fault.

This fixes the math.

While here, fix the last @i increment as well.

Fixes: 1ad9f0a464fe "target/ppc: Fix KVM-HV HPTE accessors"
Signed-off-by: Alexey Kardashevskiy <[email protected]>
Signed-off-by: David Gibson <[email protected]>