Actually will use GB directly instead, No need for MB.
User can still use libvirt GUI in MB (default) if he want to adjust the VM memory.
Source:
Valid units are “b” or “bytes” for bytes, “KB” for kilobytes (103 or 1,000 bytes), “k” or “KiB” for kibibytes (1024 bytes), “MB” for megabytes (106 or 1,000,000 bytes), “M” or “MiB” for mebibytes (220 or 1,048,576 bytes), “GB” for gigabytes (109 or 1,000,000,000 bytes)