vmspawn: rename "qemu" specific switches to not carry the "qemu" prefix

vmspawn: rename "qemu" specific switches to not carry the "qemu" prefix

This renames a few of the switches vmspawn takes, such as --qemu-mem=
and --qemu-smp= to names without the "qemu" moniker and uses less
cryptic names (i.e. --ram= and --cpus=).

I think it's a bit unsystematic that so far we use the "qemu" prefix for
some options but not for others. At least I could not figure out a
system when we use it and when we don't. Hence let's clean it up and
just use simpler names without suffix.

After all we might want to plug other hypervisors behind vmspawn one
day, hence I think there's value in sticking to generic names for these
switches that allow us to switch out backends easily. In particular for
--ram= and --cpus= which are probably the most fundamental of VM settings
there are.

The old switches are support for compat, but not advertised in man page
or --help text anymore.

I left "--qemu-gui" under its current name, since it fundamentally is a
a qemu concept, exposing a qemu specific graphical UI.