net/vhost: fix xstats after clearing stats

net/vhost: fix xstats after clearing stats

The PMD API allows stats and xstats values to be cleared separately.
This is a problem for the vhost PMD since some of the xstats values are
derived from existing stats values.  For example:

testpmd> show port xstats all
...
tx_unicast_packets: 17562959
...
testpmd> clear port stats all
...
show port xstats all
...
tx_unicast_packets: 18446744073709551615
...

Modify the driver so that stats and xstats values are stored, updated,
and cleared separately.

Fixes: 4d6cf2ac93dc ("net/vhost: add extended statistics")
Cc: stable@dpdk.org
Signed-off-by: David Christensen <drc@linux.vnet.ibm.com>
Reviewed-by: Chenbo Xia <chenbo.xia@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>