tests/libqtest: Print error instead of aborting when env variable is missing

tests/libqtest: Print error instead of aborting when env variable is missing

When you currently try to run a test directly from the command line
without setting the QTEST_QEMU_BINARY environment variable first,
you are presented with an unhelpful assertion message like this:

 ERROR:tests/libqtest.c:163:qtest_init_without_qmp_handshake:
 assertion failed: (qemu_binary != NULL)
 Aborted (core dumped)

Let's replace the assert() with a more user friendly error message
instead.

Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>