qemu_driver.c: use g_autofree when possible

String and other scalar pointers an be auto-unref, sparing us
a VIR_FREE() call.

This patch uses g_autofree whenever possible with strings and
other scalar pointer types.
Suggested-by: NErik Skultety <eskultet@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>