qemu_driver.c: use g_autoptr() when possible

Several pointer types can be auto-unref for the great majority
of the uses made in qemu_driver, sparing us a virObjectUnref()
call.

This patch uses g_autoptr() in the following pointer types inside
qemu_driver.c, whenever possible:

- qemuBlockJobDataPtr
- virCapsPtr
- virConnect
- virDomainCapsPtr
- virNetworkPtr
- virQEMUDriverConfigPtr
Suggested-by: NErik Skultety <eskultet@redhat.com>
Signed-off-by: NDaniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>