When vhostuser interfaces are used, the interface statistics
are not available in /proc/net/dev.

This change looks at the openvswitch interfaces statistics
tables to provide this information for vhostuser interface.

Note that in openvswitch world drop/error doesn't always make sense
for some interface type. When these informations are not available we
set them to 0 on the virDomainInterfaceStats.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>