avocado.virt.defaults: Add guest_user and guest_password

As this will be a user configurable option.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado: Add --guest-user and --guest-password options

Users may want to use different images, with different
setups. Allow them to override the remote login defaults
that the virt plugin assumes.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.virt.qemu.machine: .login_remote() method update

Make it honor arguments passed in the test runner command
line.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
docs: API updates and new GuestConfiguration session

Update to the latest API state and cover how to set
up custom linux guest images.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #4 from lmr/add-options-sprint14

Add options Sprint 14