KVM: selftests: Rename vm_vaddr_unused_gap() => vm_unused_gva_gap()

KVM: selftests: Rename vm_vaddr_unused_gap() => vm_unused_gva_gap()

Now that KVM selftests use gva_t instead of vm_vaddr_t, rename the API
for finding an unused range of virtual memory to drop the defunct
terminology and use "vm" for the scope.

Opportunistically clean up the function comment to drop superfluous
and redundant information.

No functional change intended.

Link: https://patch.msgid.link/20260420212004.3938325-13-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>