Device custom properties: Cache device XML after hotplug

We now cache new device XML after diskHotplug and nicHotplug,
just like we cache device XML when they are created during
vmCreate.

Change-Id: I1d7a78b6b9cff6484b97d42560092292b38d02c4
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/13802
Reviewed-by: Dan Kenigsberg <danken@redhat.com>