qemu: Improve qemuDomainDefaultSCSIControllerModel()

qemu: Improve qemuDomainDefaultSCSIControllerModel()

Make the helper stateless. This requires the caller to check
whether it needs to be called in the first place instead of
adding this check inside the function, which makes for more
readable, if a little more verbose, code.

We also update callers to check the return value against
VIR_DOMAIN_CONTROLLER_MODEL_SCSI_DEFAULT instead of a
functionally equivalent, but semantically less meaningful,
check for whether the return value is negative.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>