core: Validate FreeSpaceCriticalLowInGB conf value

Added a validation that this conf value must be non-negative.

Change-Id: Id558c9bb4eeda5e9a93a933e232bc3c2914734d6
Bug-Url: https://bugzilla.redhat.com/1130030
Signed-off-by: Allon Mureinik <amureini@redhat.com>
tools: OVF storing conf values

Added OvfItemsCountPerUpdate and OvfUpdateIntervalInMinutes to
engine-config.

Change-Id: I1306ad76286e1c834e256c0813b88300647e0202
Bug-Url: https://bugzilla.redhat.com/1128949
Signed-off-by: Allon Mureinik <amureini@redhat.com>