kvm: selftests: add vcpu support for aarch64

This code adds VM and VCPU setup code for the VM_MODE_FLAT48PG mode.
The VM_MODE_FLAT48PG isn't yet fully supportable, as it defines the
guest physical address limit as 52-bits, and KVM currently only
supports guests with up to 40-bit physical addresses (see
KVM_PHYS_SHIFT). VM_MODE_FLAT48PG will work fine, though, as long as
no >= 40-bit physical addresses are used.

Signed-off-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>