qemu_conf: Fix double free problem for cfg->firmwares
qemu_conf: Fix double free problem for cfg->firmwares

cfg->firmwares still points to the original memory address after being
freed by virFirmwareFreeList(). As cfg get freed, it will be freed again
even if cfg->nfirmwares=0 which eventually lead to crash.

The patch fix it by setting cfg->firmwares to NULL explicitly after
virFirmwareFreeList() returns

Signed-off-by: Guoyi Tu<cf564f96d76ac9e62bb6d5fd60ec60083dd899bb@h3c.com>
Reviewed-by: Ján Tomko <4cab11cfb98d3c937327354a78eb07dbb6ee2bc6@redhat.com>

crobinso/libvirt,olafhering/libvirt,libvirt/libvirt,nertpinx/libvirt,nertpinx/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,nertpinx/libvirt,crobinso/libvirt,olafhering/libvirt,jfehlig/libvirt,olafhering/libvirt,libvirt/libvirt,crobinso/libvirt,jfehlig/libvirt,nertpinx/libvirt,libvirt/libvirt,libvirt/libvirt,zippy2/libvirt,crobinso/libvirt,nertpinx/libvirt,zippy2/libvirt,zippy2/libvirt