x86: ept assertions

The upper bounds on level in set_ept_pte and install_ept_entry were
just wrong. There's nothing wrong with setting an EPT PTE at the
highest (512GiB) level.

Got rid of unused error return values and replaced them with
assertions. Nobody was checking the return values, so these functions
were silently failing.

Change-Id: I4db1e1c2a0c050f5f69cce28df460b4c8ce10d1c
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20170421005004.137260-26-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>