kvmtool/run: do not overwrite /virt/init

To me kvm_setup_guest_init() behaviour looks "obviously wrong" and
unfriendly because it always overwrites /virt/init.

kvm_setup_guest_init() is also called when we are going to use this
tree as a rootfs, and without another patch ("kvmtool/run: append
cfg.kernel_cmdline at the end of real_cmdline") the user can't use
"lkvm run -p init=my_init_path". This simply means that you can not
use a customized init unless you patch kvmtool.

Change extract_file() to do nothing if the file already exists. This
should not affect do_setup() which calls kvm_setup_guest_init() only
if make_dir(guestfs_name) creates the new/empty dir.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>