domain_conf: Relax SCSI addr used check

domain_conf: Relax SCSI addr used check

In domain_conf.c we have virDomainSCSIDriveAddressIsUsed()
function which returns true or false if given drive address is
already in use for given domain config or not. However, it also
takes a shortcut and returns true (meaning address in use) if the
unit number equals 7. This is because for some controllers this
is reserved address. The limitation comes mostly from vmware and
applies to lsilogic, buslogic, spapr-vscsi and vmpvscsi models.
On the other hand, we were not checking for the maximum unit
number (aka LUN number) which is also relevant and differs from
model to model.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>