df: Minimize the number of times we launch the libguestfs appliance.

This commit greatly improves the performance of the 'virt-df'
command by batching as many disks as possible onto a single appliance.
In many situations this means the appliance is launched only once,
versus one launch per domain as before.

However doing it this way is a lot more complex:

(1) Because of limits in Linux and virtio-blk, we can only attach
26 disks maximum at a time to the appliance.

(2) We have to use LVM filters (lvm-set-filter) to confine LVM to
the disks of a single guest.

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