qmp-test: Drop dependence on global_qtest

qmp-test: Drop dependence on global_qtest

As a general rule, we prefer avoiding implicit global state
because it makes code harder to safely copy and paste without
thinking about the global state.  Although qmp-test does not
maintain parallel qtest connections, it was the last test
assigning to global_qtest.  It's just as easy to be explicit
about the state; once all tests have been cleaned up, a later
patch can then get rid of global_qtest and a layer of wrappers
in libqtest.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>