The folder "qga/qapi-generated" shows up after building QEMU, and
gets in the way during e.g. "git add ."; Add it to .gitignore to
keep it from accidentally ending up in the wrong place.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>