Add helper functions for mounted file system resizing operations

There is a lot of commonality and code repetition for resizing of file
systems which can only be resized while mounted.  Resizing of btrfs, jfs
and xfs all follow the pattern: mkdir, mount, resize, umount and rmdir.
Copying an xfs file system also uses a similar pattern, but for the
source and destination xfs file systems simultaneously.

Add three helper functions to the FileSystem class which implement
common tasks, allowing mounted file system resizing to be implemented
more simply.

Also add a function to the Utils class which checks whether the kernel
supports a file system.  It handles the case of non-loaded modules,
which currently leads to reporting the growing of jfs and xfs as
unsupported.