Enable mypy on nova/utils.py

As a follow up for a review comment in [1] this patch enables mypy for
nova/utils, fixes the existing mypy findings, and adds some trivial type
annotations where make sense.

[1]https://review.opendev.org/c/openstack/nova/+/956089/comment/caec94ed_4fdb16bf/

Change-Id: I29ca69bd1e583adc1b1f408bd45de183649986d2
Signed-off-by: Balazs Gibizer <gibi@redhat.com>

pyproject.toml

@@ -137,6 +137,7 @@ files = [
     "nova/scheduler/client/report.py",
     "nova/scheduler/request_filter.py",
     "nova/scheduler/utils.py",
+    "nova/utils.py",
     "nova/virt/driver.py",
     "nova/virt/hardware.py",
     "nova/virt/libvirt/machine_type_utils.py",