xen/balloon: find non-conflicting regions to place hotplugged memory

xen/balloon: find non-conflicting regions to place hotplugged memory

Instead of placing hotplugged memory at the end of RAM (which may
conflict with PCI devices or reserved regions) use allocate_resource()
to get a new, suitably aligned resource that does not conflict.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
v3:
- Remove stale comment.

(cherry picked from commit 55b3da98a40dbb3776f7454daf0d95dde25c33d2)
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>