engine: VMs from manual pool cannot be started

A manual pool VM after shutdown cannot be started again
problem fixed.

Another check for RunVmCommand added, testing if the VM is part of
a manual pool. In case stateless snapshots exist, they are
not removed before the VM start.

Change-Id: I41403c46811188989d3a6c42fec708c0780b0e20
Signed-off-by: Libor Spevak <lspevak@redhat.com>