Fix wrong description about minimum values

The following options have minimum values defined, and too small values
are not rounded but rejected by oslo.config.

This change updates the description to explain the actual behavior.

Closes-Bug: #2007532
Change-Id: I8d1533ae4b44d4e8f811dce554196f270e25da3e

Author: kajinamit

nova/conf/libvirt.py

@@ -324,11 +324,11 @@
 Target maximum period of time Nova will try to keep the instance paused
 during the last part of the memory copy, in *milliseconds*.
 
-Will be rounded up to a minimum of 100ms. You can increase this value
-if you want to allow live-migrations to complete faster, or avoid
-live-migration timeout errors by allowing the guest to be paused for
-longer during the live-migration switch over. This value may be exceeded
-if there is any reduction on the transfer rate after the VM is paused.
+Minimum downtime is 100ms. You can increase this value if you want to allow
+live-migrations to complete faster, or avoid live-migration timeout errors
+by allowing the guest to be paused for longer during the live-migration switch
+over. This value may be exceeded if there is any reduction on the transfer rate
+after the VM is paused.
 
 Related options:
 
@@ -340,7 +340,7 @@
                help="""
 Number of incremental steps to reach max downtime value.
 
-Will be rounded up to a minimum of 3 steps.
+Minimum number of steps is 3.
 """),
     cfg.IntOpt('live_migration_downtime_delay',
                default=75,