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>

README

@@ -18,6 +18,10 @@ This invocation runs the msr test case. The test outputs to stdio.
 Using qemu (supported since qemu 1.3):
 qemu-system-x86_64 -enable-kvm -device pc-testdev -serial stdio -device isa-debug-exit,iobase=0xf4,iosize=0x4 -kernel ./x86/msr.flat
 
+Note that it's not necessary to specify the "-m" option to qemu. The default
+memory size is enough. Actually, the tests infrastructure doesn't support too
+much RAM anyway, so specifying a large amount of RAM may break it.
+
 Or use a runner script to detect the correct invocation:
 ./x86-run ./x86/msr.flat
 To select a specific qemu binary, specify the QEMU=<path> environment: