KVM: TDX: Move TDX hardware setup from main.c to tdx.c

KVM: TDX: Move TDX hardware setup from main.c to tdx.c

Move TDX hardware setup to tdx.c, as the code is obviously TDX specific,
co-locating the setup with tdx_bringup() makes it easier to see and
document the success_disable_tdx "error" path, and configuring the TDX
specific hooks in tdx.c reduces the number of globally visible TDX symbols.

Reviewed-by: Kai Huang <kai.huang@intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250523001138.3182794-2-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>