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

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

This reverts commit 0c2e6a6c2da4207912ac8956a38ef4997c29efb2.

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 46cfa1b2e455
("x86/PCI: Enable a 64bit BAR on AMD Family 15h (Models 00-1f, 30-3f,
60-7f)") also got amended by the following 21362ddab ("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>