[VM] Can vm-visit-virtual-folder offer a default when used interactively?

[kosowskyj]

vm-visit-folder and vm-visit-imap-folder both offer a default based on the most 
recent visit.

It would be nice (and consistent) if vm-visit-virtual-folder had a similar 
default option. The relevant interactive code could be borrowedfrom 
vm-visit-imap-folder using a new variable vm-last-visit-virtual-folder instead 
of vm-last-visit-imap-folder.

Similar changes should be made of course to 
vm-visit-virtual-folder-other-frame/window.

To do this most robustly, it might be helpful to add the possibility of adding 
an optional default field to the function vm-read-string (analogous to 
read-string); alternatively, one could use read-string instead of vm-read-string