NVMe drive cannot be shrunk.

Since commit c80d8b06 we can use the @exact parameter (set
to false) to return success if the block device is larger than
the requested offset (even if we can not be shrunk).

Use this parameter to implement the NVMe truncate() coroutine,
similarly how it is done for the iscsi and file-posix drivers
(see commit 82325ae5 "Evaluate @exact in protocol drivers").

Reported-by: Xueqiang Wei <xuwei@redhat.com>
Suggested-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20201210125202.858656-1-philmd@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>