libsel4camkes: Set virtqueue size on creation

The queue size of a virtqueue is how many entries its rings and
descriptor table have.

virtqueue: Add volatile qualifers and casts

The common initialiastion function is used by both the virtqueues and
the msgqueues. The virtqueues will supply a volatile buffer but can be
casted down. This commit mainly addresses compiler warnings from the
virtqueue side.

libsel4camkes: Move some VQ funcs to common file

This commit moves some common virtqueue functions into a common location
that can be shared by other files. Also change the virtqueue memory
allocator so that it can use other block sizes.