virtme-run: Add real support for kernel arguments

Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Add basic networking support

Now you can do virtme-run --net to get working QEMU userspace
networking.

This intentionally does not use the kernel's built-in DHCP support:
lots of kernels don't configure it, and I doubt it supports DNS.

Signed-off-by: Andy Lutomirski <luto@amacapital.net>