[Issue 276] QEMU Agent configuration settings

[mpywell]

Extend QEMU agent handling by introducing the qemu_quest_agent block

qemu_guest_agent adds the ability to set each of the QEMU Guest Agent's available settings:

• Enable/Disable the agent (replaces the qemu_agent bool)
• Setting the agent type
• Toggling freeze/thaw filesystems on backup
• Toggling fstrim after disk move or VM migration

Each added setting defaults to Proxmox's backend defaults when not configured.

Closes #276

[mpywell]

Hi @lbajolet-hashicorp have rerolled this one and added comments in the review

[DerLinkman]

This would be really nice to have merged :)