As a part of a series of larger changes the default synced folder type
accidentally got wired up to DefaultProvider instead of its dedicated
defaultSyncedFolderType() method.
This was working fine for "virtualbox" where the provider name and the
synced folder name are both the same, but it was causing virtualbox
synced folders to be selected when using the "docker" provider and
making things break.
This is one necessary step to get machine lifecycles working again with
Docker.
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