Yes, I have NVIDIA card. Thank you Alex for confirming the issue and the
tip about compiling the driver.
Personally, unless I find other more major issues, I rather skip using
suspend instead of switching to a proprietary driver.
I am also running Ubuntu on the same machine and there suspend is
working fine so I guess this must be a Fedora/Xen/Qubes issue.
I also notice sometimes when getting back from having the screen locked
for a while that the screen is not repainted properly and sometimes it
is necessary to Alt+F2,Alt+F1 switch to an other virtual terminal and
back to get it to work again. However, so far that has worked everytime
it happens so I can live with that as well for now.