Added support for emulating block devices in the virtual filesystem

Added constant for the BLOCK filetype in vfsStreamContent.

Added new file: vfsStreamBlock

vfsStreamBlock is an extension of vfsStreamFile to represent a block device in the virtual filesystem, much the same way block devices are actually represented.

Added method to get a new block device in vfsStream.

Added a case for block devices in vfsStreamAbstractVisitor.

If there are any other requirements needed for this to be merged upstream please let us know, block device support is something that we found beneficial for our testing purposes.
When I started the project I always added them, but my personal style has changed to not have them any more. Now, when I change a file and stumble about one I remove it, but I don't force a removal at the moment.