README: add information about memory usage

I got a report of someone trying to run tests with a large amount of
RAM (4GB), which broke the guest as free_memory() function (called
by setup_vm()) will override the PCI hole.

Let's document memory constraints so that people don't do that.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>