snapshot: simplify redefinition of disk snapshot

snapshot: simplify redefinition of disk snapshot

Redefining disk-only snapshot xml should work even if the user
did not explicitly pass VIR_DOMAIN_SNAPSHOT_CREATE_DISK_ONLY;
the flag is only required for conditions where the <state>
subelement is not already present in parsing (that is, defining
a new snapshot).

Also, fix the error code of some user-visible errors (the remaining
VIR_ERR_INTERNAL_ERROR should not be user-visible, since parsing
of <active> is only done from internal code).

* src/conf/domain_conf.c (virDomainSnapshotDefParseString): Allow
disks during redefinition of disk snapshot.