qemu: Avoid overflow when setting migration speed on inactive domains
    
    Commit c4206d7 fixed the overflow for running domains. However, we need
    a similar check when setting migration speed on inactive domains.
    
    At first look, it may seem the check in c4206d7 is now redundant but
    qemuDomainMigrateSetMaxSpeed is not the only caller of
    qemuMonitorSetMigrationSpeed so we need to check the bandwidth in both
    places.
    
    https://bugzilla.redhat.com/show_bug.cgi?id=1083483
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>