qemu_vm.py: Minor changes

Minor changes in conditions and comments

Signed-off-by: Vaclav Ehrlich <vehrlich@redhat.com>
usb_redirection.py: Migration support added

Added migration support in thread.
Some minor changes.

Signed-off-by: Vaclav Ehrlich <vehrlich@redhat.com>
tests-spice.cfg: Added usb redirection support

Signed-off-by: Vaclav Ehrlich <vehrlich@redhat.com>