vmspawn: initialize block device "serials" from backing file name

vmspawn: initialize block device "serials" from backing file name

If we pass multiple block devices into a VM it's really useful to pass
recognizable serial numbers on them, so that we know which one is which.
qemu allows setting them, hence initialize them automatically from the
filename of the backing file, as a convenience feature.

Inside of a VM this means /dev/disk/by-id/… symlinks will be generated
with useful identifiers.