move vmt(4) from MD to MI, and add support vmt on aarch64. tested on ESXi-Arm Fling

- move from sys/arch/x86/x86/{vmt.c,vmtreg.h,vmtvar.h} to sys/dev/vmt/{vmt_subr.c,vmtreg.h,vmtvar.h},
and split the attach part of the cpufeaturebus and fdt
- add aarch64 vmware backdoor op
- add include guard to vmt{reg,var}.h
- Yet there is still some little-endian dependency. it needs to be fixed in order to work properly on aarch64eb