[libvirt] Do not fail migration if the media is ejected inside guest.

[Osier Yang]

The whole mail title should be "Do not fail migration if the media is
ejected inside guest and the media source is removed".

Following 2 patches are trying to solve it, please see the patch
descriptions for more detail.

BZ# https://bugzilla.redhat.com/show_bug.cgi?id=725673

[PATCH 1/2] qemu: Get the media status of removable block device
[PATCH 2/2] qemu: Clear source path of removable block if it is

Osier