qemuxml2argv: Mock virSCSIDeviceGetSgName

qemuxml2argv: Mock virSCSIDeviceGetSgName

When constructing SCSI hostdev command line for qemu, the
/sys/bus/scsi/devices/... dir is scanned. Unfortunately, even in
the tests. This is needed to determine the name of SCSI device to
passthrough to qemu, because in the domain XML we were given its
address instead. Anyway, we should not be touching live system
data in our test suite as it produced unpredictable results. The
test is regressing from 1e9a083742efe on.

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