The default is C<NULL> unless overridden by setting
C<LIBGUESTFS_APPEND> environment variable.
-The string C<append> is stashed in the libguestfs handle, so the caller
-must make sure it remains valid for the lifetime of the handle.
-
Setting C<append> to C<NULL> means I<no> additional options
are passed (libguestfs always adds a few of its own).
The default is C<$libdir/guestfs> unless overridden by setting
C<LIBGUESTFS_PATH> environment variable.
-The string C<path> is stashed in the libguestfs handle, so the caller
-must make sure it remains valid for the lifetime of the handle.
-
Setting C<path> to C<NULL> restores the default path.
=head2 set-qemu | qemu
You can also override this by setting the C<LIBGUESTFS_QEMU>
environment variable.
-The string C<qemu> is stashed in the libguestfs handle, so the caller
-must make sure it remains valid for the lifetime of the handle.
-
Setting C<qemu> to C<NULL> restores the default qemu binary.
=head2 set-verbose | verbose