KVM: selftests: Fix alignment in virt_arch_pgd_alloc() and vm_vaddr_alloc()

Refactor virt_arch_pgd_alloc() and vm_vaddr_alloc() in both RISC-V and
aarch64 to fix the alignment of parameters in a couple of calls. This will
make it easier to fix the alignment in a future commit that adds an extra
parameter (that happens to be very long).

No functional change intended.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
Reviewed-by: Andrew Jones <andrew.jones@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20221017195834.2295901-9-ricarkol@google.com