libqos/qgraph: add qos_node_create_driver_named()

libqos/qgraph: add qos_node_create_driver_named()

So far the qos subsystem of the qtest framework had the limitation
that only one instance of the same official QEMU (QMP) driver name
could be created for qtests. That's because a) the created qos
node names must always be unique, b) the node name must match the
official QEMU driver name being instantiated and c) all nodes are
in a global space shared by all tests.

This patch removes this limitation by introducing a new function
qos_node_create_driver_named() which allows test case authors to
specify a node name being different from the actual associated
QEMU driver name. It fills the new 'qemu_name' field of
QOSGraphNode for that purpose.

Adjust build_driver_cmd_line() and qos_graph_node_set_availability()
to correctly deal with either accessing node name vs. node's
qemu_name correctly.

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <3be962ff38f3396f8040deaa5ffdab525c4e0b16.1611704181.git.qemu_oss@crudebyte.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>