KVM: Remove kvm_vcpu_gfn_to_page() and kvm_vcpu_gpa_to_page()

Drop helpers to convert a gfn/gpa to a 'struct page' in the context of a
vCPU.  KVM doesn't require that guests be backed by 'struct page' memory,
thus any use of helpers that assume 'struct page' is bound to be flawed,
as was the case for the recently removed last user in x86's nested VMX.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220429010416.2788472-8-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>