CA-347560: Add VM.import_metadata_async
CA-347560: Add VM.import_metadata_async

This is a variant of `VM.import_metadata` that always queues the operation and
returns a task id immediately (like most VM operations). This is useful, once
the original (synchronous) function may block for longer periods while other
operations on the VM complete.

Signed-off-by: Rob Hoes <8783b3376fb07d39edffc8d7da68a6de3874a7e6@citrix.com>

xapi-project/message-switch,xapi-project/message-switch,xapi-project/message-switch