avocado.plugins: Introduce virt plugin

Introduce the virt plugin, that adds virtualization
specific options to the avocado test runner. The first
two options are:

--qemu-bin: Provide a specific qemu bin to be tested
--qemu-dst-bin: Provide a specific qemu as a migration destination

More options will be added later.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.virt: Add VirtTest class

This test class will be able to filter the virt specific
options from the avocado command line options and use
it in tests. Eventually, convenience methods should be
added to the class.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
tests/qemu: Use the specialized VirtTest class

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins.builtin: Fix plugin loading
avocado.plugins: Add virt_bootstrap plugin

Add a plugin to help the user with bootstraping his/her
virt testing environment. It is very simple ATM, and
its only purpose right now is to download the JeOS
20 x86_64 image.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>