refactor: split hyperlight_vm into shared and x86_64-specific code

Extract x86_64-specific methods (new, initialise,
dispatch_call_from_host, reset_vcpu, get_root_pt,
get_snapshot_sregs, handle_debug, crashdump_context)
and the debug submodule into hyperlight_vm/x86_64.rs.

Keep shared code in mod.rs: struct definition, error types,
helper functions, and architecture-independent methods
(map_region, unmap_region, run, handle_io, etc.).

Struct fields changed from private to pub(super) so the
x86_64 submodule can access them.

Signed-off-by: Ludvig Liljenberg <4257730+ludfjig@users.noreply.github.com>

Author: ludfjig