e1000e: Prevent crash from legacy interrupt firing after MSI-X enable

A race condition between guest driver actions and QEMU timers can lead
to an assertion failure when the guest switches the e1000e from legacy
interrupt mode to MSI-X. If a legacy interrupt delay timer (TIDV or
RDTR) is active, but the guest enables MSI-X before the timer fires,
the pending interrupt cause can trigger an assert in
e1000e_intmgr_collect_delayed_causes().

This patch removes the assertion and executes the code that clears the
pending legacy causes. This change is safe and introduces no unintended
behavioral side effects, as it only alters a state that previously led
to termination.

- when core->delayed_causes == 0 the function was already a no-op and
  remains so.

- when core->delayed_causes != 0 the function would previously
  crash due to the assertion failure. The patch now defines a safe
  outcome by clearing the cause and returning. Since behavior after
  the assertion never existed, this simply corrects the crash.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/1863
Suggested-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Message-ID: <20250807110806.409065-1-lvivier@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
(cherry picked from commit 8e4649cac9)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

hw/net/e1000e_core.c

@@ -336,11 +336,6 @@ e1000e_intmgr_collect_delayed_causes(E1000ECore *core)
 {
     uint32_t res;
 
-    if (msix_enabled(core->owner)) {
-        assert(core->delayed_causes == 0);
-        return 0;
-    }
-
     res = core->delayed_causes;
     core->delayed_causes = 0;