Beryllium virus detected on bootcamp partition, cannot resolve
The pagefile.sys file is used by Windows as virtual memory. Basically, Avast running under macOS is giving you a false positive and thus can be ignored. I tested this by performing a clean install of Windows 10 (20H2) in a VMware Fusion Player (12.1.1) virtual machine. Next, I installed Avast and found no malware. However, I wanted Avast to scan pagefile.sys for viruses. Below is the procedure I used.

I restarting Windows to boot to the Windows Recovery Environment. Basically, I held down the shift key while restarting Windows.

I navigated to a Windows Command Prompt window.

I entered the following commands. Basically, I wanted a copy of pagefile.sys.
c:
attrib -h -s pagefile.sys
copy pagefile.sys pagefile.bin
attrib +h +s pagefile.sys
exit


I booted back to Windows.

Using the File Explorer, I selected to run a scan on pagefile.bin, as shown below. For a better view, click on the image or open the image in a new window.

The result is shown below.



So Avast under Windows is ignoring the pagefile.sys file, but does not under macOS. Being this test was made from a clean install of Windows, I assume this found malware threat is a false positive and can therefore be ignored.