virDomainSnapshotAlignDisks: refactor extension to all disks

virDomainSnapshotAlignDisks: refactor extension to all disks

Last step of the algorithm in virDomainSnapshotAlignDisks is to extend
the array of disks to all VM's disk and provide defaults. This was done
by extending the array, adding defaults at the end and then sorting it.
This requires the 'idx' variable and also a separate sorting function.

If we store the pointer to existing snapshot disk definitions in a hash
table and create a new array of snapshot disk definitions, we can fill
the new array directly by either copying the definition from the old
array or adding the default.

This avoids the sorting step and thus even the need to store the index
of the domain disk altogether.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>