qemu_driver.c: use g_auto* in some functions

qemu_driver.c: use g_auto* in some functions

This patch changes qemuDomainSnapshotLoad, qemuDomainCheckpointLoad and
qemuStateInitialize to use g_autoptr() and g_autofree, cleaning up
some virObjectUnref() and VIR_FREE() calls on each.

The reason this is being sent separately is because these are not
trivial search/replace cases. In all these functions some strings
declarations are moved inside local loops, where they are in fact
used, allowing us to erase VIR_FREE() calls that were made inside
the loop and in 'cleanup' labels.

Following patches with tackle more trivial cases of g_auto* usage
in all qemu_driver.c file.

Suggested-by: Ján Tomko <jtomko@redhat.com>
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>