nVMX: Add helper to check if a memtype is supported for EPT structures

Add a helper to check if a given memtype can be used for EPT structures,
and use the helper to clean up the EPT test code.  An informational
message is lost along the way, but that's not necessarily a bad thing.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211125012857.508243-36-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>