Throw error when starting a container with invalid virtiofs source (#1051)

Run = Create + Start

1) Mount source points to a valid directory
- Run and Create + Start both correctly create the container with mount.

2) Mount source points to a file
   - Run fails bootstrapping the container, thus container not created.
- Create creates the container, but Start fails bootstrapping, removing
the container. (Thus, both are the same.)

3) Mount source deleted or replaced to file after container created
   - Start throw errors but do not delete the container.

Sources/ContainerCommands/Container/ContainerStart.swift

@@ -18,6 +18,7 @@ import ArgumentParser
 import ContainerAPIClient
 import ContainerizationError
 import ContainerizationOS
+import Foundation
 import TerminalProgress
 
 extension Application {
@@ -69,6 +70,12 @@ extension Application {
                 return
             }
 
+            for mount in container.configuration.mounts where mount.isVirtiofs {
+                if !FileManager.default.fileExists(atPath: mount.source) {
+                    throw ContainerizationError(.invalidState, message: "path '\(mount.source)' is not a directory")
+                }
+            }
+
             do {
                 let io = try ProcessIO.create(
                     tty: container.configuration.initProcess.terminal,