avocado.virt.qemu.machine: Creates screendumps dir only when necessary.

Avoid to create the screendumps directory inside the test result
when not required, just when you pass the proper command line
argument to create the screendumps.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Merge pull request #22 from ruda/fix_screendump_dir_point

Creates screendumps directory only when necessary.