KVM: selftests: Rename vm_vaddr_populate_bitmap() => vm_populate_gva_bitmap()

KVM: selftests: Rename vm_vaddr_populate_bitmap() => vm_populate_gva_bitmap()

Now that KVM selftests use gva_t instead of vm_vaddr_t, rename the helper
for populating the initial GVA bitmap to drop the defunct terminology and
use "vm" for the scope.

Opportunistically fixup the declaration of the API, which has been broken
since day 1.  The flaw went unnoticed because the sole caller is defined
after the weak version, i.e. can see the prototype without a previous
declaration.

No functional change intended.

Fixes: e8b9a055fa04 ("KVM: arm64: selftests: Align VA space allocator with TTBR0")
Link: https://patch.msgid.link/20260420212004.3938325-14-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>