ppc: Huge page detection mechanism fixes - Episode III

After already fixing two issues with the huge page detection mechanism
(see commit 159d2e39a860 and 86b50f2e1bef), Greg Kurz noticed another
case that caused the guest to crash where QEMU announces huge pages
though they should not be available for the guest:

qemu-system-ppc64 -enable-kvm ... -mem-path /dev/hugepages \
 -m 1G,slots=4,maxmem=32G
 -object memory-backend-ram,policy=default,size=1G,id=mem-mem1 \
 -device pc-dimm,id=dimm-mem1,memdev=mem-mem1 -smp 2 \
 -numa node,nodeid=0 -numa node,nodeid=1

That means if there is a global mem-path option, we still have
to look at the memory-backend objects that have been specified
additionally and return their minimum page size if that value
is smaller than the page size of the main memory.

Reported-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Greg Kurz <groug@kaod.org>
Tested-by: Greg Kurz <groug@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
