qemu.test: add support mirror image to nfs/iscsi

New test to mirroring block device to nfs server/iscsi disk

Signed-off-by: Xu Tian <xutian@redhat.com>
libvirt tests: PEP8 Fixes

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
virttest.qemu_devices: PEP8 Fixes

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>