KVM: selftests: Unconditionally use memslot 0 for vaddr allocations

Drop the memslot param(s) from vm_vaddr_alloc() now that all callers
directly specific '0' as the memslot.  Drop the memslot param from
virt_pgd_alloc() as well since vm_vaddr_alloc() is its only user.
I.e. shove the hardcoded '0' down to the vm_phy_pages_alloc() calls.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>