kargs: parse spaces in kargs input and keep quotes

kargs: parse spaces in kargs input and keep quotes

According to Jonathan's suggestion, should fix the code from
ostree repo.

With this patch:
- kargs input like "init_on_alloc=1 init_on_free=1", will be
parsed as 2 seperated args `init_on_alloc=1` and `init_on_free=1`,
instead of whole;
- According to https://www.kernel.org/doc/html/v4.14/admin-guide/kernel-parameters.html,
need to keep spaces in double-quotes, like `param="spaces in here"`
will be parsed as whole instead of 3.

Fixes https://github.com/coreos/rpm-ostree/issues/4821