providers/virtualbox: extract reading dhcpservers from hostonlyifs

This is just a refactor, no behavior change.

Instead of stitching together dhcpserver info in the structure returned
from `read_host_only_interfaces`, sprout a new driver method called
`read_dhcp_servers` to return that information separately.

This means that driver clients (well there's really only _one_ client in
`ProviderVirtualBox::Action::Network`) have to do a bit more work to get
interface and DHCP server information.

But this gives us (a) a cleaner and more consistent driver interface and
(b) groundwork for a fix for #3083, which will require interacting with
DHCP servers outside of the context of host-only interfaces.