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

release/scripts/mk-vmimage.sh

@@ -93,15 +93,7 @@ main() {
 		. "${VMCONFIG}"
 	fi
 
-	case ${TARGET}:${TARGET_ARCH} in
-		arm64:aarch64)
-			ROOTLABEL="ufs"
-			NOSWAP=1
-			;;
-		*)
-			ROOTLABEL="gpt"
-			;;
-	esac
+	ROOTLABEL="gpt"
 
 	vm_create_base
 	vm_install_base

release/tools/vmimage.subr

@@ -32,9 +32,9 @@ write_partition_layout() {
 			;;
 		arm64:aarch64)
 			ESP=yes
-			SCHEME=mbr
+			SCHEME=gpt
 			BOOTPARTS=
-			ROOTFSPART="-p freebsd:=${VMBASE}"
+			ROOTFSPART="-p freebsd-ufs/rootfs:=${VMBASE}"
 			;;
 		powerpc:powerpc*)
 			ESP=no