From: Roman Bogorodskiy
Since 5.1.0, it's possible to pass additional command-line
+arguments to the bhyve process when starting the domain using the
+<bhyve:commandline> element under domain.
+To supply an argument, use the element <bhyve:arg> with
+the attribute value set to additional argument to be added.
+The arg element may be repeated multiple times. To use this XML addition, it is necessary
+to issue an XML namespace request (the special xmlns:name attribute)
+that pulls in http://libvirt.org/schemas/domain/bhyve/1.0;
+typically, the namespace is given the name of bhyve.
+
Example:
++<domain type="bhyve" xmlns:bhyve="http://libvirt.org/schemas/domain/bhyve/1.0"> + ... + <bhyve:commandline> + <bhyve:arg value='-somebhyvearg'/> + </bhyve:commandline> +</domain>