Simplify live memory resize for KVM

[paczerny]

Description
The live memory resize was implemented in #1660 , using virsh attach-device described here. But this implementation has a lot of limitations, especially for removing the memory.
To remove the limitation, use virsh setmem as described here

Use case
Easier live memory resizing for kvm

Interface Changes
None.

Additional Context
New implementation doesn't need the MEMORY_SLOTS attribute

I haven't find which QEMU version is needed for this implementation

Progress Status

• Branch created
• Code committed to development branch
• Sunstone Testing - QA
• Documentation
• Release notes - resolved issues, compatibility, known issues
• Code committed to upstream release/hotfix branches
• Documentation committed to upstream release/hotfix branches

[paczerny]

PRs:

• sources
• docs remove memory slots
• docs update resolved issues

[rsmontero]

There are some valid use cases for the hotplug method (e.g. PCI passtrhough devices to allow DMA, improved support across guest OS...).

This feature is to include a MEMORY_RESIZE_MODE:

• balloon the actual implementation
• hotplug the previous one