* Add --mount parameter for "whitelisting" guest_paths and defining
  their respective filesystem paths on the host FS
* Add UhyveFileMap structure
* Add sandbox support to open() syscall

A few points that could be further worked are unit tests, handling
more of the parsing using the clap library directly and performance
optimizations.

Helped-by: Çağatay Yiğit Şahin <[email protected]>
Helped-by: Jonathan Klimt <[email protected]>

Also removes the "Introduction" header, as the header takes more
space than the text itself now.

Originally introduced in e7869a9.

Introduces a test for UhyveFileMap. Some refactoring was done so
as to keep the two tests, fs-test and uhyvefilemap, in separate
files. Some filesystem-related functions were moved into common.rs,
because:
- we anticipate that they will be necessary for further
  filesystem-related tests
- putting two test functions in a single test (e.g. fs-test)
  causes the second test to hang for some mysterious reason
- more descriptive errors

- Rename --file_map to --mount
- Temporarily remove short parameter
- Temporarily remove environment variable
- Don't split file_map params with commas
- Change documentation, remove references to file_map variable