qemu: move temp file of screenshot and memorypeek to per-domain dir

qemu: move temp file of screenshot and memorypeek to per-domain dir

The temp files of screenshot and memory peek, which are created by QEMU,
are put in the cache directory.  However, the caches of domain
capabilities, which are created and used by libvirtd, are also put in
the cache directory.  In order to make the cache directory more secure,
move the temp files of screenshot and memory peek to per-domain
directory.

Since the temp files are just temporary files and are only used by
libvirtd (libvirtd will delete them after use), the use of screenshot
and memory peek will be affected.

Signed-off-by: Peng Liang <liangpeng10@huawei.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>