tests/qtest/e1000e-test: Check qemu_recv() succeeded

tests/qtest/e1000e-test: Check qemu_recv() succeeded

The e1000e_send_verify() test calls qemu_recv() but doesn't
check that the call succeeded, which annoys Coverity. Add
an explicit test check for the length of the data.

(This is a test check, not a "we assume this syscall always
succeeds", so we use g_assert_cmpint() rather than g_assert().)

Fixes: Coverity CID 1432324
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Message-id: 20210525134458.6675-3-peter.maydell@linaro.org