[TESTS] Explicitly specify 256M RAM when launching Xvisor on QEMU

QEMU by default only provides 128MB of RAM and there would be many
cases (e.g. more than one guest) where Xvisor would require RAM
greater than 128MB.

Currenlty, for all supported ARM host emulated by QEMU have atleast
256MB hence, with this patch update documentation to explicitly
specify 256M RAM when launching Xvisor on QEMU.

Signed-off-by: Anup Patel <anup@brainfault.org>