xenfb.c: avoid expensive loops when prod <= out_cons

xenfb.c: avoid expensive loops when prod <= out_cons

If the frontend sets out_cons to a value higher than out_prod, it will
cause xenfb_handle_events to loop about 2^32 times. Avoid that by using
better checks at the beginning of the function.

upstream-commit-id: ac0487e1d2ae811cd4d035741a109a4ecfb013f1

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Reported-by: Ling Liu <liuling-it@360.cn>