Ensure non-existent machines do not attempt to list snapshots

Prior to this commit, if a snapshot restore was run on an entire
environment with some non-existent guests, Vagrant would attempt to list
their snapshots with a nil id. This commit fixes that by returning an
empty list of snapshots if the machine has not been created yet.

plugins/providers/virtualbox/cap.rb

@@ -29,6 +29,7 @@ module VagrantPlugins
       #
       # @return [Array<String>] Snapshot Name
       def self.snapshot_list(machine)
+        return [] if machine.id.nil?
         machine.provider.driver.list_snapshots(machine.id)
       end
     end

test/unit/plugins/providers/virtualbox/cap_test.rb

@@ -40,4 +40,19 @@ describe VagrantPlugins::ProviderVirtualBox::Cap do
       expect(described_class.forwarded_ports(machine)).to be(nil)
     end
   end
+
+  describe "#snapshot_list" do
+    it "returns all the snapshots" do
+      allow(machine).to receive(:id).and_return("1234")
+      allow(driver).to receive(:list_snapshots).with(machine.id).
+        and_return(["backup", "old"])
+
+      expect(described_class.snapshot_list(machine)).to eq(["backup", "old"])
+    end
+
+    it "returns empty array when the machine is does not exist" do
+      allow(machine).to receive(:id).and_return(nil)
+      expect(described_class.snapshot_list(machine)).to eq([])
+    end
+  end
 end