avocado.virt: Introduce templates

Introduce templates to create custom qemu command line invocations.
The templates are native Python string templates with custom
attributes that comes from the devices.py API.

The current attributes are:

* {qemu_bin}
* {avocado_fd_default}
* {avocado_qmp_default}
* {avocado_display_default}
* {avocado_vga_default}
* {avocado_drive_default}
* {avocado_net_default}
* {avocado_serial_default}
* {avocado_migration}
* {avocado_defaults}

The default template is stored in defaults.py.
Samples available in examples/*.tpl.

Here's a valid template:

{qemu_bin}
	-sandbox on
	{avocado_defaults}

The invocation from the command line:

$ avocado run --qemu-template example.tpl tests/qemu/boot.py

Signed-off-by: Rudá Moura <rmoura@redhat.com>
This complex example looks great!