default-configs: Enable CONFIG_VMXNET3_PCI only on x86

The device can not be instantiated on many non-x86 and just prints
some error messages, e.g.:

$ qemu-system-ppc64 -device vmxnet3 -M g3beige
[vmxnet3][WR][vmxnet3_init_msix]: Failed to initialize MSI-X, error -95
[vmxnet3][WR][vmxnet3_pci_realize]: Failed to initialize MSI-X, configuration is inconsistent.

Since vmxnet3 is a para-virtualized device that is only useful on x86,
it should also only be enabled on the x86 targets.

Reviewed-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
