arm64: convert virtual machine images to GPT

These images were switched to MBR in r281876 as a way to cope with a
hard-coded partition GUID in QEMU's default EFI firmware. Enough time
has passed that this is no longer a problem; QEMU versions >= 4.0
include a copy of edk2 EFI firmware that can detect the root filesystem
properly. Alternatively, sysutils/u-boot-qemu-arm64 can be used.

Switch back to building these images with a GPT partition table, and
re-enable the swap partition.

Reviewed by: gjb, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D26986