The VM doesn't use X when communicating with the host, so cut/past can't work.
I put it in a file and use scp to copy it. I also use ssh to log into the box from which I can use X for cut/past and windows from the VM.
Fdisk should list the partitions known. The /dev/mapper are virtual disks/partitions composed of partitions already existing.
In my case, things show up as:
# fdisk -l
Disk /dev/vda: 17.2 GB, 17179869184 bytes
16 heads, 63 sectors/track, 33288 cylinders, total 33554432 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0001a906
Device Boot Start End Blocks Id System
/dev/vda1 * 2048 1026047 512000 83 Linux
/dev/vda2 1026048 9414655 4194304 82 Linux swap / Solaris
/dev/vda3 9414656 33554431 12069888 83 Linux
I get vda from using xen with only one disk defined.
Adding swap (if you want a new disk) has to be done via the virtualbox control to define it. Otherwise you are just creating a swap file, which can be put on any mounted disk. And yes, the filesystem it is on must be mounted before it can be added as a swap device.