KVM: selftests: Rename translate_to_host_paddr() => translate_hva_to_hpa()

KVM: selftests: Rename translate_to_host_paddr() => translate_hva_to_hpa()

Rename arm64's translate_to_host_paddr() to translate_hva_to_hpa() and
update variable names to match, as using "vaddr" and "paddr" terminology
is super confusing due to selftests using those exact names for *guest*
addresses.

Opportunisitically drop superfluous local page_addr and paddr variables.

No functional change intended.

Link: https://patch.msgid.link/20260420212004.3938325-15-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>