PVH: support whitespace cmdline splitting

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.

Reviewed by: imp
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D36190