Add ability to grow cloned vm disks

[rmb938]

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Description

When using proxmox clone allow the ability to resize existing disks after the clone

Use Case(s)

When using packer we typically want to perform updates, install new software, ect... this can require more disk space then the existing VM template has. Currently one has to pre-grow the template disks first which depending on the storage format could cause a bunch of extra space usage for no reason.

Potential configuration

disks {
    type = "scsi"
    existing_index = 0
    disk_size = "10G"
  }

Potential References