x86/vmx: fix detection of unmapped PTE

check_ept_ad() was testing get_ept_pte() for 0, which meant that the PTE
lookup failed.  dff740c0 ("x86: ept assertions") changed the return
value in that case to -1, which broke the test.

Returning 0 and -1 is ambiguous, so let's return false instead and get
the PTE through a pointer argument.  This skips a test that was failing
before, because it was looking at invalid type (the meta -1) instead of
the pte.

Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>