x86: Allow to limit maximum RAM address

While there is a feature to limit RAM memory, we should also be able to
limit the maximum RAM address. Specifically, svm can only work when the
maximum RAM address is lower than 4G, as it does not map the rest of the
memory into the NPT.

Allow to do so using the firmware, when in fact the expected use-case is
to provide this infomation on bare-metal using the MEMLIMIT parameter in
initrd.

Signed-off-by: Nadav Amit <namit@vmware.com>
Message-Id: <20200710183320.27266-5-namit@vmware.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>