conf: Format managed property of hostdev-pci ports correctly

conf: Format managed property of hostdev-pci ports correctly

The property is parsed using virTristateBoolTypeFromString() but
formatted as if it was a regular bool, which results in the
following incorrect conversion:

  BOOL_ABSENT -> managed='no'
  BOOL_YES    -> managed='yes'
  BOOL_NO     -> managed='yes'

Use the virTristateBoolTypeToString() helper to ensure the
setting can survive a roundtrip conversion.

Fixes: 4b4a981d60d3372fb73bb3546ab1052844b2e3d5
Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>