Add ability to do VM full disk copy

[Jaydeehow]

Satisfies issue #37. Makes the disk copy a full copy instead of fast copy if copy_type = "full" is specified, and storage_repository = "" are specified in the .tf file. Example:

// Create a new virtual machinewith a full disk copy resource "xenserver_vm" "fullCopyVM" { base_template_name = "centos7-minimal-template" name_label = "JDH_terraform_fullcopy" static_mem_min = 2147483648 static_mem_max = 2147483648 dynamic_mem_min = 2147483648 dynamic_mem_max = 2147483648 vcpus = 2 boot_order = "c" copy_type = "full" storage_repository = "Local storage" hard_drive { is_from_template = true user_device = "0" } # Template VM HDD cdrom { is_from_template = true user_device = "3" } }