avocado.virt.qemu.machine: Fix serial console not being recorded

We should remove the serial console file only when the VM
instance is closed.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #20 from lmr/fix-serial-console

avocado.virt.qemu.machine: Fix serial console not being recorded
avocado.virt.test: Removes duplicate entry for guest_password.

Removes duplicate entry when testing for argument job.guest.guest_password.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
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>
avocado.virt.qemu.machine.VM: Empty arguments will not raise exception later.

Allows to use avocado.virt.qemu.machine.VM with empty parameters,
so you can make use of it in command line, like:

>>> import avocado.virt.qemu import machine
>>> vm = machine.VM()
>>> vm.power_on()
>>> vm.power_off()

without raising exception of kind 'NoneType' object and 'get' attribute.

Signed-off-by: Rudá Moura <rmoura@redhat.com>
Merge pull request #23 from ruda/fix_machine_vm_params

 mpty arguments for avocado.virt.qemu.machine.VM class will not raise exception later.
Merge pull request #21 from ruda/fix_dup_guest_password

Removes duplicate entry for guest_password.
Merge pull request #22 from ruda/fix_screendump_dir_point

Creates screendumps directory only when necessary.