vmm: variant or associated item `KernelMmap` not found for this enum

[peppergrayxyz]

Build fails on arch64 (#429):

• default_guest_memory is x86_64 only
• create_guest_memory signature has changed and the call is broken on aarch64

error[E0599]: no variant or associated item named `KernelMmap` found for enum `builder::Payload` in the current scope
    --> src/vmm/src/builder.rs:2186:68
     |
500  | enum Payload {
     | ------------ variant or associated item `KernelMmap` not found for this enum
...
2186 |         create_guest_memory(mem_size_mib, &vm_resources, &Payload::KernelMmap)
     |                                                                    ^^^^^^^^^^ variant or associated item not found in `builder::Payload`

error[E0308]: arguments to this function are incorrect
    --> src/vmm/src/builder.rs:2225:13
     |
2225 |             create_guest_memory(128, None, Payload::Empty).unwrap();
     |             ^^^^^^^^^^^^^^^^^^^            -------------- expected `&Payload`, found `Payload`
     |
note: expected `&VmResources`, found `Option<_>`
    --> src/vmm/src/builder.rs:2225:38
     |
2225 |             create_guest_memory(128, None, Payload::Empty).unwrap();
     |                                      ^^^^
     = note: expected reference `&resources::VmResources`
                     found enum `Option<_>`
note: function defined here
    --> src/vmm/src/builder.rs:1379:4
     |
1379 | fn create_guest_memory(
     |    ^^^^^^^^^^^^^^^^^^^
1380 |     mem_size: usize,
1381 |     vm_resources: &VmResources,
     |     --------------------------
1382 |     payload: &Payload,
     |     -----------------
help: consider borrowing here
     |
2225 |             create_guest_memory(128, None, &Payload::Empty).unwrap();
     |                                            +

error[E0308]: mismatched types
    --> src/vmm/src/builder.rs:2224:13
     |
2224 |         let (guest_memory, _arch_memory_info) =
     |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected a tuple with 4 elements, found one with 2 elements
2225 |             create_guest_memory(128, None, Payload::Empty).unwrap();
     |             ------------------------------------------------------- this expression has type `(vm_memory::GuestMemoryMmap, arch::ArchMemoryInfo, shm::ShmManager, builder::PayloadConfig)`
     |
     = note: expected tuple `(vm_memory::GuestMemoryMmap, arch::ArchMemoryInfo, shm::ShmManager, builder::PayloadConfig)`
                found tuple `(_, _)`