run: Avoid machine_type setting when using custom config

When custom config was used and default_guest_os was None,
the _process_machine_type added a DEFAULT_MACHINE_TYPE filter,
which might fail on configs without 'i440fx' variant.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>
virt: Adds migration tests for dst problem.

1) Migration to read-only space.
2) Migration to low space destination.

Both migration uses exec protocol for migration to file.

qemu-kvm is started like Expect class not only like Tail.

Signed-off-by: Jiří Župka <jzupka@redhat.com>
qemu_vm: Log the original traceback when VM.create() failes

When self.make_create_command() raises an exception in VM.create()
it's catched and reraised. The problem is that the information about
actual failed line is lost. This patch logs the original traceback
and raise VMCreateError instead.

Signed-off-by: Lukáš Doktor <ldoktor@redhat.com>