core: Introduce host status Kdumping

Adds Kdumping status to VDSStatus to identify host is currently dumping
its kernel crash. This status will be used in following patches.

Change-Id: I1361baed86f8d062d29d7e75dae116125ca26ed3
Bug-Url: https://bugzilla.redhat.com/1079821
Signed-off-by: Martin Perina <mperina@redhat.com>