torture: Remove qemu dependency on EFI firmware

On some (probably misconfigured) systems, the torture-test scripting
will cause qemu to complain about missing EFI firmware, often because
qemu is trying to traverse broken symbolic links to find that firmware.
Which is a bit silly given that the default torture-test guest OS has
but a single binary for its userspace, and thus is unlikely to do much
in the way of networking in any case.

This commit therefore avoids such problems by specifying "-net none"
to qemu unless the TORTURE_QEMU_INTERACTIVE environment variable is set
(for example, by having specified "--interactive" to kvm.sh), in which
case "-net nic -net user" is specified to qemu instead.  Either choice
may be overridden by specifying the "-net" argument of your choice to
the kvm.sh "--qemu-args" parameter.

Link: https://lore.kernel.org/lkml/20190701141403.GA246562@google.com


Reported-by: Joel Fernandes <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de>