tests/libqos/e1000e: Make e1000e libqos functions independent from global_qtest

libqos library functions should never depend on functions (like memread(),
memwrite() or clock_step()) that require global_qtest to be set, since
library functions might get used in qtests that track multiple states, too.
Thus let's replace the global_qtest-related functions with their independent
counterparts.

Message-Id: <20190904130047.25808-3-thuth@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>