Description of problem:
I clicked 'Add Hardware'.

Version-Release number of selected component:
virt-manager-1.1.0-4.git310f6527.fc21

Additional info:
reporter:       libreport-2.3.0
backtrace_rating: 4
cmdline:        /usr/bin/python2 -tt /usr/share/virt-manager/virt-manager
crash_function: gtk_css_image_linear_equal
executable:     /usr/bin/python2.7
kernel:         3.18.7-200.fc21.x86_64
runlevel:       N 5
type:           CCpp
uid:            1000

Truncated backtrace:
Thread no. 1 (10 frames)
 #0 gtk_css_image_linear_equal at gtkcssimagelinear.c:558
 #1 gtk_css_value_array_equal at gtkcssarrayvalue.c:97
 #2 _gtk_css_computed_values_get_difference at gtkcsscomputedvalues.c:272
 #3 _gtk_style_context_validate at gtkstylecontext.c:3077
 #11 gtk_container_idle_sizer at gtkcontainer.c:1738
 #15 g_signal_emit_by_name at gsignal.c:3405
 #16 gdk_frame_clock_paint_idle at gdkframeclockidle.c:408
 #22 g_main_context_iteration at gmain.c:3842
 #23 g_application_run at gapplication.c:2282
 #24 ffi_call_unix64 at ../src/x86/unix64.S:76