qemu: pass pointers instead of copying objects for qemuFirmware*FreeContent()

qemu: pass pointers instead of copying objects for qemuFirmware*FreeContent()

These functions all cooperate to free memory pointed to by a single
object that contains (doesn't *point to*, but actually contains)
several sub-objects. They were written to send copies of these
sub-objects to subordinate functions, rather than just sending
pointers to the sub-objects.

Let's change these functions to just send pointers to the objects
they're cleaning out rather than all the wasteful and pointless
copying.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>