This how Linux works. It has nothing to do with Oracle VM.
Why in the world would you want your VM guest to actively use a swap file? One of the greatest assets to any linux environment is linux will not use a page file unless it is absolutely necessary. If you have an oracle database actively forcing writes to a swap file... .then reduce your overall memory allocation to the database.
I'd just as soon read the block from disk again instead of swaping it out a page file.
Are you dedicating a disk or disks to just handle "swapping"? If not, then realize all of it is coming out of the same io path.
Yes. You must specify a page file with a Oracle Database. I don't even use the minimum required and ignore the error message on the prereq check. Also, if for some reason you do run out of physical memory.... linux will crash if no swap file is used. However, if you have something causing paging..... In my opinion you should check into it.