Add 'FromCache' to virDomainVirtioSerialAddrAutoAssign
Commit 19a148b dropped the cache from QEMU's private domain object.
Assume the callers do not have the cache by default and use
a longer name for the internal ones that do.
This makes the shorter 'virDomainVirtioSerialAddrAutoAssign'
name availabe for a function that will not require the cache.