KVM: selftests: Drop "vaddr_" from APIs that allocate memory for a given VM

KVM: selftests: Drop "vaddr_" from APIs that allocate memory for a given VM

Now that KVM selftests use gva_t instead of vm_vaddr_t, drop "vaddr_" from
the core memory allocation APIs as the information is extraneous and does
more harm than good.  E.g. the APIs don't _just_ allocate virtual memory,
they allocate backing physical memory and install mappings in the guest
page tables.  And as proven by kmalloc() and malloc(), developers generally
expect that allocations come with a working virtual address.

Opportunistically clean up the function comment for vm_alloc(), and drop
the misleading and superfluous comments for its wrappers.

No functional change intended.

Link: https://patch.msgid.link/20260420212004.3938325-12-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>