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.
@mikey179 unrelated question: does your CS allow ending php tags?