src: Check for virDomainDiskInsert() retval properly

src: Check for virDomainDiskInsert() retval properly

Our coding style specifies that only negative values are considered as
error. Check for return value of virDomainDiskInsert() properly,
following the style. Not that the function can now return anything other
than 0 or -1, but it just triggers my OCD.

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