This bug has been addressed by Miroslav Rezanina <mrezanin@redhat.com>, with the following series:
[RHEL-7.5.z qemu-guest-agent PATCH 0/7] Add querry commands necessary for vdsm

Fix in qemu-guest-agent-2.8.0-2.el7_5.1

I suggest you remove the mention of utmpx.h from the doc_text.

Verified bug with qemu-guest-agent-2.8.0-2.el7_5.1.

1)qemu-guest-agent test run
https://polarion.engineering.redhat.com/polarion/#/project/RedHatEnterpriseLinux7/testrun?id=virtqe-qemu-guest-agent-RHEL7-5-z%20-%20qemu-guest-agent-RHEL7-5-z%20guest

no found regression and blocker bug.

2) new agent command work.

{"execute": "guest-get-osinfo"}
{"return": {"name": "Red Hat Enterprise Linux Server", "kernel-release": "3.10.0-862.el7.x86_64", "version": "7.5 (Maipo)", "variant": "Server", "pretty-name": "Red Hat Enterprise Linux Server 7.5 (Maipo)", "version-id": "7.5", "variant-id": "server", "kernel-version": "#1 SMP Wed Mar 21 18:14:51 EDT 2018", "machine": "x86_64", "id": "rhel"}}

{"execute": "guest-get-host-name"}
{"return": {"host-name": "vm-74-10.lab.eng.pek2.redhat.com"}}

{"execute": "guest-get-users"}
{"return": []}

{"execute": "guest-get-timezone"}
{"return": {"zone": "CST", "offset": 28800}}

3. automation test pass.  Result is in attachment.

Created attachment 1471006 [details]
automation result

Base on comment7 and comment8, set this bug as verified.

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-2018:2460