Overhaul: Qemu Guest Agent

[Tinyblargon]

Fixes #323

Re-implements the QemuGuestAgent as the current version of proxmox has more options than disable and enable:

type QemuGuestAgent struct {
	Enable *bool               `json:"enable,omitempty"` // Optional
	Type   *QemuGuestAgentType `json:"type,omitempty"`   // Optional
	Freeze *bool               `json:"freeze,omitempty"` // Optional
	FsTrim *bool               `json:"trim,omitempty"`   // Optional
}

I'll fix this downstream in the Terraform provider as well.