vhost-user-test: use correct ROM to speed up and avoid spurious failures

The mechanism to get the option ROM for virtio-net does not block the
PCI ROM from being loaded. Therefore, in vhost-user-test there are
two entries in the boot menu for the virtio-net card: one as an
embedded option ROM, one from the ROM BAR.

The embedded option ROM in vhost-user-test is the non-EFI-enabled,
while the ROM BAR has an EFI-enabled ROM. The two are compiled with
slightly different parameters, where only the old BIOS-only one doesn't
have a timeout for the "Press Ctrl-B" banner. When using a new
machine type, therefore, the vhost-user-test has to wait for the
EFI-enabled ROM's banner to go away. There are several ways to fix
this:

1) fix the ROMs to have the same configuration

2) add ",romfile=" to the -device line

3) remove --option-rom and add the ROM file name to the -device line

4) use an old machine type

This patch chooses 3. In addition, the file name was wrong because
qtest runs QEMU relative to the top build directory, not to the
x86_64-softmmu/ subdirectory, which is fixed too.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>