KVM: x86/mmu: Add typedefs for rmap/iter handlers

Add typedefs for the MMU handlers that are invoked when walking the MMU
SPTEs (rmaps in legacy MMU) to act on a host virtual address range.

No functional change intended.

Cc: Ben Gardon <bgardon@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210226010329.1766033-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>