kvm: selftests: introduce new VM mode for 64K pages

Rename VM_MODE_FLAT48PG to be more descriptive of its config and add a
new config that has the same parameters, except with 64K pages.

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