avocado.virt.qemu: Introduce qemu templates.

Introduce qemu templates to create custom command line invocations.
The templates are native Python string templates with custom
tags that comes from values of `avocado.virt.qemu.devices` API.

The current tags are:

* {avocado_defaults}  all options, with qemu binary path included
* {avocado_qemu_binary}  the qemu binary path
* {avocado_devices}  the devices, without the qemu binary path
* {avocado_qmp}  qmp device
* {avocado_display}  display device
* {avocado_vga}  vga device
* {avocado_drive} drive device
* {avocado_network}  network device
* {avocado_serial}  serial device
* {avocado_vnc}  vnc device
* {avocado_fd}  floppy disk device

Some tags accepts to get internal values. For example,
from `avocado_drive` you can get the default disk image
with `{avocado_drive.drive_file}`.

Samples available in examples/*.tpl.

Here's a valid template:

```
{avocado_qemu_binary}
	-sandbox on -m 64
	{avocado_devices}
```

The invocation from the command line:

`avocado run --qemu-template example.tpl tests/qemu/boot.py`

Signed-off-by: Rudá Moura <rmoura@redhat.com>