netfilter: add a netbuffer filter

This filter is to buffer/release packets. Can be used when using
MicroCheckpointing or other Remus like VM FT solutions.
You can also use it to crudely simulate network delay.  Doesn't
actually delay individual packets, but batches them together, which is
a delay of sorts.

Usage:
 -netdev tap,id=bn0
 -object filter-buffer,id=f0,netdev=bn0,queue=rx,interval=1000

NOTE:
 Interval is in microseconds, it can't be omitted currently, and can't be 0.

Signed-off-by: Yang Hongyang <yanghy@cn.fujitsu.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>