bhyve: support interface type 'network'

bhyve: support interface type 'network'

Add support for interface type 'network'. While bridge remains the only
supported options for networks in bhyve, supporting interface type
'network' allows easier configuration and makes domain XMLs more
compatible with the other drivers.

While here, update the error message for the unsupported interface type
to print the requested network type string instead of an integer to make
it more user-friendly.

Signed-off-by: Roman Bogorodskiy <bogorodskiy@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>