qemu: Allow spaces in disk serial

qemu: Allow spaces in disk serial

https://bugzilla.redhat.com/show_bug.cgi?id=1195660

There's been a bug report appearing on the qemu-devel list, that
libvirt is unable to pass spaces in disk serial number [1]. Not only
our RNG schema forbids that, the code is not prepared either. However,
with a bit of escaping (if needed) we can allow spaces there.

1: https://lists.gnu.org/archive/html/qemu-devel/2015-02/msg04041.html

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
(cherry picked from commit 5aee81a0cb60308b8614ee37c0b69c1dde1f00d8)