qemuPrepareNVRAM: Save domain conf only if domain's persistent

qemuPrepareNVRAM: Save domain conf only if domain's persistent

In one of my previous patches (3a3c3780b) I've tried to fix the
problem of nvram path disappearing on a domain that's been
started and shut down again. I fixed this by explicitly saving
domain's config file.  However, I did a bit of clumsy without
realizing we have a transient domains for which we don't save the
config file. Hence, any domain using UEFI became persistent.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
(cherry picked from commit 54ddc08ddb3d86e90b48b655a51577761e20ac34)