Revert "xen/balloon: Mark unallocated host memory as UNUSABLE"

This reverts commit b3cf8528.

That commit unintentionally broke Xen balloon memory hotplug with
"hotplug_unpopulated" set to 1. As long as "System RAM" resource
got assigned under a new "Unusable memory" resource in IO/Mem tree
any attempt to online this memory would fail due to general kernel
restrictions on having "System RAM" resources as 1st level only.

The original issue that commit has tried to workaround fa564ad9
("x86/PCI: Enable a 64bit BAR on AMD Family 15h (Models 00-1f, 30-3f,
60-7f)") also got amended by the following 03a55173

 ("x86/PCI: Move
and shrink AMD 64-bit window to avoid conflict") which made the
original fix to Xen ballooning unnecessary.

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>