These are the packages that would be merged, in order:

Calculating dependencies ... done!

Oops! Conflicts have been encountered:

>>> dev-python/protobuf-python-3.4.1:0/14::python-kit, ebuild scheduled for merge, wants ~dev-libs/protobuf-3.4.1
>>> sys-process/criu-3.4:0/0::nokit, installed, wants dev-libs/protobuf[python]
>>> dev-libs/protobuf-c-1.3.0-r1:0/1.0.0::dev-kit, installed, wants dev-libs/protobuf:0/13=

My candidates are:

>>> dev-libs/protobuf-3.4.1:0/14::dev-kit, ebuild scheduled for merge has SLOT 0/14 but not IUSE python
>>> dev-libs/protobuf-3.3.0:0/13::dev-kit, installed has SLOT 0/13 and IUSE python

We hope this informational output has been useful in identifying the
problem. We are continually working to reduce conflicts. Do not
hesitate to report a bug at https://bugs.funtoo.org. Thank you!

emerge: there are no ebuilds to satisfy "~dev-python/ovs-2.7.2[python_targets_python2_7(-)?,python_targets_python3_4(-)?,python_targets_python3_5(-)?,-python_single_target_python2_7(-),-python_single_target_python3_4(-),-python_single_target_python3_5(-)]".
(dependency required by "net-misc/openvswitch-2.8.1::net-kit" [ebuild])
(dependency required by "@selected" [set])
(dependency required by "@world" [argument])

Seems like dev-python/protobuf-python wants net-libs/protobuf:0/14 while net-libs/protobuf:0/13 is needed for dev-libs/protobuf-c. Also seems like the build is failing because due to no USE="python" in the dev-libs/protobuf:0/14 ebuild.