xen/balloon: only hotplug additional memory if required

Now that we track the total number of pages (included hotplugged
regions), it is easy to determine if more memory needs to be
hotplugged.

Add a new BP_WAIT state to signal that the balloon process needs to
wait until kicked by the memory add notifier (when the new section is
onlined by userspace).

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
---
v3:
- Return BP_WAIT if enough sections are already hotplugged.

v2:
- New BP_WAIT status after adding new memory sections.