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"
}
}