Allow querying of variable number of parameters

This patch adds support for querying in gnt-instance list of:
  - disk.count
  - nic.count
  - disk.size/$N
  - nic.(ip|mac|bridge)/$N

The patch also disables the exception raised when the header description
is missing, that should be fixed in the future (together with better
handling of variable numeric fields).

Reviewed-by: imsnah