docs: bhyve: document commandline element
docs: bhyve: document commandline element

Document the <bhyve:commandline> element which allows
to inject custom command line arguments for bhyve.

Signed-off-by: Roman Bogorodskiy <2a54bf90b2b53e2ab35ffe11dda2730a23b4aa9c@gmail.com>
Reviewed-by: John Ferlan <87558058f6f829e5ec976c8ef960720af4ff9c7d@redhat.com>

eskultety/libvirt,zippy2/libvirt,jfehlig/libvirt,crobinso/libvirt,libvirt/libvirt,jardasgit/libvirt,andreabolognani/libvirt,zippy2/libvirt,eskultety/libvirt,eskultety/libvirt,eskultety/libvirt,jardasgit/libvirt,jfehlig/libvirt,andreabolognani/libvirt,zippy2/libvirt,olafhering/libvirt,nertpinx/libvirt,andreabolognani/libvirt,libvirt/libvirt,fabianfreyer/libvirt,nertpinx/libvirt,jfehlig/libvirt,crobinso/libvirt,olafhering/libvirt,nertpinx/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,crobinso/libvirt,fabianfreyer/libvirt,andreabolognani/libvirt,olafhering/libvirt,libvirt/libvirt,libvirt/libvirt,nertpinx/libvirt,jardasgit/libvirt,andreabolognani/libvirt,jardasgit/libvirt,fabianfreyer/libvirt,eskultety/libvirt,olafhering/libvirt,crobinso/libvirt,fabianfreyer/libvirt,zippy2/libvirt,nertpinx/libvirt,jfehlig/libvirt