[libvirt] storage conf: Add key-value options to storage pools

[Wido den Hollander]

This series of patches adds the ability to pass down options to the
storage pool drivers.

In the case of NFS users can specify mount options and in the case of
RBD users can specify options for librados to influence some behavior.

All options and values are checked on input validity to prevent injection
of malicious commands.

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Wido den Hollander