KVM: VMX: Use GPA legality helpers to replace open coded equivalents

Replace a variety of open coded GPA checks with the recently introduced
common helpers.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210204000117.3303214-6-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>