Apply scaling down the calculated guest ram size to the number of pages

Calculate the guest ram size based a ratio proportional to the
number of pages available, rather than the amount of memory
available in bytes, in the host. This is to ensure that the
result is always page-aligned.

If the result of get_ram_size() isn't aligned to the host page
size, it triggers an error in __kvm_set_memory_region(), called
via the KVM_SET_USER_MEMORY_REGION ioctl, which requires the size
to be page-aligned.

Fixes: 18bd8c3b ("kvm tools: Don't use all of host RAM for guests by default")
Signed-off-by: Fuad Tabba <tabba@google.com>
Link: https://lore.kernel.org/r/20230717121232.3559948-4-tabba@google.com


Signed-off-by: Will Deacon <will@kernel.org>