Add functionality to execute scripts on Xen host prior to booting and exporting the VM

[dongyuzheng]

Use case: Setting and removing GPU passthroughs

• pre_boot_host_scripts (array of strings) - List of scripts to execute on the
  XenServer host prior to booting the VM. The VM's UUID will be passed into each
  script as the first parameter. The script can be of any type: bash, python, ruby, etc.
  Each script must start with a shebang, ie. #!/usr/bin/env bash.

• pre_export_host_scripts (array of strings) - List of scripts to execute on the
  XenServer host prior to exporting the VM. The VM's UUID will be passed into each
  script as the first parameter. The script can be of any type: bash, python, ruby, etc.
  Each script must start with a shebang, ie. #!/usr/bin/env bash.

• remote_ssh_port (integer) - SSH port of XenServer host. Only useful when using
  pre_boot_host_scripts or pre_export_host_scripts. Default is 22.