avocado.plugins.virt: Make app_args attribute check before access

Sometimes the Namespace object will not have the specific
attributes, because that option wasn't passed in the CLI.
Be more careful to avoid the plugin not being activated.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.plugins.virt_bootstrap: Change message priorities

Change message priorities for better output. Also, re-phrase
a number of messages for clarity.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado-virt: Introduce a namespace for the plugin variables

Instead of setting flat, global variables for the options
passed to the avocado command line, introduce a namespace -
avocado.args.run. Examples:

    avocado.args.run.qemu_bin
    avocado.args.run.qemu_dst_bin
    avocado.args.run.guest_user

This will reduce the chance of variable clashes inside
plugin code to virtually zero. Change all references
to those variables, and add the missing qemu_bin and
qemu_io command line options to the command line, so
we make all options to have an equivalent in the command
line.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #7 from lmr/variables-namespace

Variables namespace