Fix spice includes
    
    Spice release version 0.31 requires that only spice-client.h or
    spice-client-gtk.h should be included directly. As a result,
    compilation is now throwing warnings like:
    
    warning: #warning "Only <spice-client.h> can be included directly" [-Wcpp]
    warning: #warning "Only <spice-client-gtk.h> can be included directly" [-Wcpp]
    
    This patch also bumps spice version requirement to 0.31, to ensure
    those files are available.
    
    Signed-off-by: Eduardo Lima (Etrunko) <etrunko@redhat.com>