qemuBuildMemoryBackendStr: Handle one more corner case

qemuBuildMemoryBackendStr: Handle one more corner case

https://bugzilla.redhat.com/show_bug.cgi?id=1458638

This code is so complicated because we allow enabling the same
bits at many places. Just like in this case: huge pages can be
enabled by global <hugepages/> element under <memoryBacking> or
on per <memory/> basis. To complicate things a bit more, users
are allowed to omit the page size which case the default page
size is used. And this is what is causing this bug. If no page
size is specified, @pagesize is keeping value of zero throughout
whole function. Therefore we need yet another boolean to hold
[use, don't use] information as we can't sue @pagesize for that.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>