tests/qtest: Add a function that checks whether a device is available

Devices might not always be compiled into the QEMU target binaries.
We already have the libqos framework that is good for handling such
situations, but some of the qtests are not a real good fit for the
libqos framework. Let's add a qtest_has_device() function for such
tests instead.

Message-Id: <20211220081054.151515-2-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>