LibVirt machinery should create the KVM domain

Currently, LibVirt machinery can determine the snapshot and revert
the VM state to it, but it jumps straight to waiting for a state
change without actually issuing a create call to the domain being
used. This is likely due to it expecting a live-state snapshot to
be instantiated which automatically starts the instance.

Add a conditional call self.vms[label].create() after resolving VM
snapshot state and prior to entering the wait state. If the VM
power state is still off, try to create the KVM domain with basic
error handling. This should not case race conditions as preceeding
snapshot operations are serialized and complete prior to returning.

lib/cuckoo/common/abstracts.py

@@ -392,6 +392,14 @@ def start(self, label):
             self._disconnect(conn)
             raise CuckooMachineError("No snapshot found for virtual machine "
                                      "{0}".format(label))
+        # Attempt to boot the VM (create domain) if it's powered off
+        # This should address use of offline snapshots for fast clones/other uses.
+        if self._status(label) == self.POWEROFF:
+            try:
+                self.vms[label].create()
+            except libvirt.libvirtError:
+                raise CuckooMachineError("Unable to create domain for "
+                                         "virtual machine {0}".format(label))
 
         # Check state.
         self._wait_status(label, self.RUNNING)