Currently, `db_opts[:local_file]` will always be a FIFO with the new
file_storage mechanisms, so `.validate_free_space` will only check
against the tmp dir, which usually isn't very large and not the target
destination for the DB backup/dump.

This fix moves the check to a place were we can get access to the
file_storage object directly and target the directory of the mounted
filesystem that will receive the backup/dump.

To that end, the check now only applies to file storage classes that are
"mountable", since checking against non-mountable storages don't make
sense (s3/swift don't have this capability, and they are assumed to be
"infinite" anyway, and I don't think you can query FTP for available
storage as well so I think the same assumption is also fair to apply)

* * *

Quick note:

In a couple of spots in the tests, the following is added:

    allow(file_storage).to receive(:class).and_return(MiqSmbSession)

Despite `file_storage` being defined above with:

    let(:file_storage) { double("MiqSmbSession", :disconnect => nil) }

This is now required after moving validate_free_space to inside the
with_file_storage method, and wrapping it in a class check of:

    if file_storage.class <= MiqGenericMountSession

This "stubs the stub" to have it act like a MiqSmbSession, when in fact,
it isn't.  Unfortunately, this can't be used globally since it causes a
bunch of other tests failures.