virsh: clean up attach-interface paragraph in man page

virsh: clean up attach-interface paragraph in man page

This makes the paragaph about attach-interface more descriptive and
correct, adding in a few bits of information that were previously
missing, e.g. --script is only allowed for bridge interfaces of Xen
domains, target name is regenerated if it starts with vnet, mac
address will be autogenerated if not specified.

(I did this in response to an email asking why a script couldn't be
specified for a bridge interface of a qemu domain, and why an
interface of type='ethernet' couldn't be created with
attach-interface)