PVH: support whitespace cmdline splitting

[cperciva]

Summary

For historical reasons, Xen kernel command lines have options
separated by commas. Every other FreeBSD platform uses whitespace;
this is also necessary in PVH in order to support the Firecracker
VMM. Allow options to be separated by any combination of commas
and whitespace.

[imp]

Though some things are like "gee=woo foo=bar,baz,bing" will now be misparsed.

[cperciva]

In D36190#821461, @imp wrote:

Though some things are like "gee=woo foo=bar,baz,bing" will now be misparsed.

Yeah, that's an unavoidable consequence of wanting to support Xen's historical use of commas as argument separators. Maybe at some point we should switch to only using commas here if we're booting under Xen, but accepting both commas and whitespace is good enough for now, I think.