configs: enable and build-in CONFIG_VIRTIO_MMIO*

Firecracker micro-vm fails to boot. It turns out that the following
configs must be enabled and set to built-in:

- CONFIG_VIRTIO_MMIO
- CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES
Signed-off-by: NCaspar Zhang <caspar@linux.alibaba.com>
Tested-by: NEryu Guan <eguan@linux.alibaba.com>

config-4.19.y-x86_64

@@ -3103,8 +3103,8 @@ CONFIG_VIRTIO_PCI=y
 CONFIG_VIRTIO_PCI_LEGACY=y
 CONFIG_VIRTIO_BALLOON=y
 # CONFIG_VIRTIO_INPUT is not set
-CONFIG_VIRTIO_MMIO=m
-# CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES is not set
+CONFIG_VIRTIO_MMIO=y
+CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES=y
 
 #
 # Microsoft Hyper-V guest support

config-4.19.y-x86_64-debug

@@ -3102,8 +3102,8 @@ CONFIG_VIRTIO_PCI=y
 CONFIG_VIRTIO_PCI_LEGACY=y
 CONFIG_VIRTIO_BALLOON=y
 # CONFIG_VIRTIO_INPUT is not set
-CONFIG_VIRTIO_MMIO=m
-# CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES is not set
+CONFIG_VIRTIO_MMIO=y
+CONFIG_VIRTIO_MMIO_CMDLINE_DEVICES=y
 
 #
 # Microsoft Hyper-V guest support