CONFIG_VIRTIO_MEM is not available for thedefaultarchitecture x86.
Result is shown for architecture 64BIT

Virtio mem driver

modulename: virtio_mem.ko

configname: CONFIG_VIRTIO_MEM

Linux Kernel Configuration
└─>Device Drivers
└─>Virtio drivers
└─>Virtio mem driver
In linux kernel since version 5.1 (release Date: 2019-05-05)  
This driver provides access to virtio-mem paravirtualized memory
devices, allowing to hotplug and hotunplug memory.

This driver was only tested under x86-64, but should theoretically
work on all architectures that support memory hotplug and hotremove.

If unsure, say M.

source code: