pkg/build/fuchsia: expand fvm image
pkg/build/fuchsia: expand fvm image

The fvm partition used was the default from the build. This commit
expands it to 2GB. Without this change, recent versions of fuchsia will
not boot.

xairy/syzkaller,xairy/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,google/syzkaller,xairy/syzkaller,google/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller,xairy/syzkaller