conf: add 'isa' controller type

conf: add 'isa' controller type

Introduce 'isa' controller type. In domain XML it looks this way:

    ...
    <controller type='isa' index='0'>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01'
               function='0x0'/>
    </controller>
    ...

Currently, this is needed for the bhyve driver to allow choosing a
specific PCI address for that. In bhyve, this controller is used to
attach serial ports and a boot ROM.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>