Add support for mount ext4 USB flash drive to WSL

As mentioned in the documentation, this feature is currently not supported with the new option wsl --mount but it is very useful when you need to access a Linux filesystem inside an SD card (Raspian for example) attached with a USB adapter.

This is the error:

PS C:\Windows\System32> wmic diskdrive list brief
Caption                          DeviceID            Model                            Partitions  Size
NVMe Samsung SSD 970             \\.\PHYSICALDRIVE0  NVMe Samsung SSD 970             3           500105249280
Generic Mass-Storage USB Device  \\.\PHYSICALDRIVE1  Generic Mass-Storage USB Device  2           32004564480

PS C:\Windows\System32> wsl --mount \\.\PHYSICALDRIVE1 --bare
Impossibile trovare l'unità specificata.
PS C:\Windows\System32>

The ext4 filesystem inside the SD card is not visible in Windows, so:

giuseppe@DESKTOP-J4Q83NF:/mnt/c/Windows/System32$ sudo mount -t drvfs D: /mnt/d
[sudo] password for giuseppe:
<4>WARNING: mount: waiting for virtio device...
<3>init: (14) ERROR: MountPlan9WithRetry:285: mount drvfsa on /mnt/d (cache=mmap,rw,msize=262144,trans=virtio,aname=drvfs;path=D:;symlinkroot=/mnt/) failed: 2
mount: No such file or directory
giuseppe@DESKTOP-J4Q83NF:/mnt/c/Windows/System32$

Thanks

Start by reviewing the documented wsl --mount behavior and the reported wsl --mount \\.\PHYSICALDRIVE1 --bare command. Investigate how WSL handles USB disks and ext4 filesystems; done means an ext4 filesystem on an attached USB or SD-card device can be mounted without the reported device-not-found error.