qemu: Handle MODEL_SCSI_{AUTO,DEFAULT} appropriately

qemu: Handle MODEL_SCSI_{AUTO,DEFAULT} appropriately

The qemuDomainGetSCSIControllerModel() function, which is
responsible for choosing a model for a SCSI controller that
didn't have one provided by the user, considers values >0 to
mean "model has been set".

Since MODEL_SCSI_AUTO == 0, this means that such a value is
considered the same as MODEL_SCSI_DEFAULT (-1). This makes
sense, as not specifying a model name or explicitly asking for
one to be automatically chosen intuitively should result in
the same behavior.

Specifically, there is no case in which a value of
MODEL_SCSI_AUTO or MODEL_SCSI_DEFAULT is encountered after the
initial controller creation: it is either replaced with an
actual model, or an error is raised.

Despite this, there are a few places in the QEMU driver where
we incorrectly treat these values as if they were actual
model names. To reduce confusion, make sure that no longer
happens.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>