vm/qemu: Allow empty qemu-args.
vm/qemu: Allow empty qemu-args.

Allow setting qemu_args to "" in the config file. This is needed
when running qemu from the qemu-devel package on FreeBSD, which
does not support the -enable-kvm option.
Without this patch, an entry "" is added to the list of command
line parameters, which breaks the starting of the qemu instances.

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