The problem here is libvirt issues one ovs-vsctl command trying to fetch all the data. However, upon error, no output is produced for libvirt to parse. Therefore we must break it into smaller pieces:

int
virNetDevOpenvswitchInterfaceStats(const char *ifname,
                                   virDomainInterfaceStatsPtr stats)
{
    ...
    cmd = virCommandNew(OVSVSCTL);
    virNetDevOpenvswitchAddTimeout(cmd);
    virCommandAddArgList(cmd, "get", "Interface", ifname,
                         "statistics:rx_errors",
                         "statistics:rx_dropped",
                         "statistics:tx_errors",
                         "statistics:tx_dropped", NULL);
    ...
}

Patch proposed upstream:

https://www.redhat.com/archives/libvir-list/2017-June/msg00913.html

I've just pushed the patch upstream:

commit edaf13565712b431c6fd66a5c4a964af7759ce68
Author:     Michal Privoznik <mprivozn>
AuthorDate: Wed Jun 14 15:23:29 2017 +0200
Commit:     Michal Privoznik <mprivozn>
CommitDate: Fri Jun 23 11:51:23 2017 +0200

    virNetDevOpenvswitchInterfaceStats: Be more forgiving when fetching stats
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1461270
    
    When fetching stats for a vhost-user type of interface, we run
    couple of ovs-vsctl commands and parse their output. However, not
    all stats exist at all times, for instance "rx_dropped" or
    "tx_errors" can be missing. Thing is, we ask for a bulk of
    statistics and if one of them is missing an error is reported
    instead of returning the rest. Since we ignore errors, we fail to
    set statistics. Fix this by asking for each piece alone.
    
    Signed-off-by: Michal Privoznik <mprivozn>

v3.4.0-165-gedaf13565

Test on below packages, the result is as expected.
libvirt-3.7.0-2.el7.x86_64
openvswitch-2.8.0-0.1.20170810git3631ed2.el7fdb.x86_64

1. Set 2 guests with vhostuser type interface, then ping each other
2. on host check that there is no rx_error, and the statistics are right.

# virsh domiflist vhost1 
Interface  Type       Source     Model       MAC
-------------------------------------------------------
vhost-user1 vhostuser  -          virtio      52:54:00:69:33:29

# virsh domiflist vhost2
Interface  Type       Source     Model       MAC
-------------------------------------------------------
vhost-user2 vhostuser  -          virtio      52:54:00:93:51:db

# virsh domifstat vhost1 vhost-user1
vhost-user1 rx_bytes 403278
vhost-user1 rx_packets 4188
vhost-user1 rx_drop 50
vhost-user1 tx_bytes 404706
vhost-user1 tx_packets 4206
vhost-user1 tx_errs 0
vhost-user1 tx_drop 0

# ovs-vsctl list interface
...
name                : "vhost-user1"
...
statistics          : {"rx_1024_to_1522_packets"=0, "rx_128_to_255_packets"=0, "rx_1523_to_max_packets"=0, "rx_1_to_64_packets"=125, "rx_256_to_511_packets"=0, "rx_512_to_1023_packets"=0, "rx_65_to_127_packets"=4082, rx_bytes=404706, rx_dropped=0, rx_errors=0, rx_packets=4206, tx_bytes=403278, tx_dropped=50, tx_packets=4188}

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHEA-2018:0704

*** Bug 1624273 has been marked as a duplicate of this bug. ***