KVM: selftests: Replace "vaddr" with "gva" throughout

KVM: selftests: Replace "vaddr" with "gva" throughout

Replace all variations of "vaddr" variables in KVM selftests with "gva",
with the exception of the ELF structures, as those fields are not specific
to guest virtual addresses, to complete the conversion from vm_vaddr_t to
gva_t.

Opportunistically use gva_t instead of u64 for relevant variables, and
fixup indentation as appropriate.

No functional change intended.

Link: https://patch.msgid.link/20260420212004.3938325-17-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>