target-ppc: Confirm That .bss Pages Are Valid

The existing code does a check to ensure that a .bss region is properly
mmap'd.  When additional mmap is required, the (guest) pages are also
validated.  However, this code has a bug: when host page size is larger
than target page size, it is possible for the .bss pages to already be
(host) mapped but the guest .bss pages may not be valid.

The check to mmap additional space is separated from the flagging of the
target (guest) pages, thus ensuring that both aspects are done properly.

Signed-off-by: Tom Musta <tommusta@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>
