storageVolCreateXMLFrom: Allow multiple accesses to origvol

storageVolCreateXMLFrom: Allow multiple accesses to origvol

When creating a new volume, it is possible to copy data into it from
another already existing volume (referred to as @origvol). Obviously,
the read-only access to @origvol is required, which is thread safe
(probably not performance-wise though). However, with current code
both @newvol and @origvol are marked as building for the time of
copying data from the @origvol to @newvol. The rationale behind
is to disallow some operations on both @origvol and @newvol, e.g.
vol-wipe, vol-delete, vol-download. While it makes sense to not allow
such operations on partly copied mirror, but it doesn't make sense to
disallow vol-create or vol-download on the source (@origvol).

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>