tests: fix usb-hcd-ehci-test compilation

tests: fix usb-hcd-ehci-test compilation

Fixes commit
e5758de4e836c3b2edc2befd904651fc6967d74f ("tests/libqtest: Make
qtest_qmp_device_add/del independent from global_qtest")

and commit
dd210749727530cdef7c335040edbf81c3c5d041 ("tests/libqtest: Use
libqtest-single.h in tests that require global_qtest").

Cc: Thomas Huth <thuth@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20190926111955.17276-2-marcandre.lureau@redhat.com>
Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>