snapshot: improve disk align checking

snapshot: improve disk align checking

There were not previous callers with require_match set to true.
I originally implemented this bool with the intent of supporting
ESX snapshot semantics, where the choice of internal vs. external
vs. non-checkpointable must be made at domain start, but as ESX
has not been wired up to use it yet, we might as well fix it to
work with our next qemu patch for now, and worry about any further
improvements (changing the bool to a flags argument) if the ESX
driver decides to use this function in the future.

* src/conf/snapshot_conf.c (virDomainSnapshotAlignDisks): Alter
logic when require_match is true to deal with new XML.