vmm: Start reconciling amd64 and arm64 copies of vmm_dev.c

Most of the code in vmm_dev.c and vmm.c can and should be shared between
amd64 and arm64 (and eventually riscv) rather than being duplicated. To
the end of adding a shared implementation in sys/dev/vmm, this patch
eliminates most of the differences between the two copies of vmm_dev.c.

• Remove an unneeded cdefs.h include.
• Simplify the amd64 implementation of vcpu_unlock_one().
• Simplify the arm64 implementation of vcpu_lock_one().
• Pass buffer sizes to alloc_memseg() and get_memseg() on arm64. On amd64 this is needed for compat ioctls, but these functions should be merged.
• Make devmem_mmap_single() stricter on arm64.

Reviewed by: corvink, jhb
Differential Revision: https://reviews.freebsd.org/D44995