libvirt: Add test module for virsh {at|de}tach-disk

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Modify virsh function for virsh {at|de}tach-disk

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
virt: Add get_disk_count() for virsh {at|de}tach-disk

Signed-off-by: Li Yang <liyang.fnst@cn.fujitsu.com>
Merge pull request #304 from liyangfnst/attach_detach_disk

Add test for virsh {at|de}tach-disk
tests.virtio_console: Add boot_nr1 test

To make sure boot_nr tests are working correctly it's necessarily to boot
(and open) the port with nr=1.

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