guest: replace TIMESTAMP_LEN with actual sized structs #3

Job	Run time
static / cppcheck	17s
tests / cmake	48s
static / formatcheck	11s
tests / makefile	47s
tests / cmake (-DGUEST_BACKEND=0)	42s
tests / makefile (GUEST_BACKEND=0)	40s
tests / cmake (-DHOST_BACKEND=0)	29s
tests / makefile (HOST_BACKEND=0)	22s
staticbuild (x86_64)	15s
rpmbuild (x86_64, almalinux:8)	37s
staticbuild (ppc64le)	20s
rpmbuild (x86_64, almalinux:9)	37s
rpmbuild (x86_64, fedora:37)	1m 7s
rpmbuild (x86_64, fedora:38)	1m 20s
rpmbuild (x86_64, fedora:rawhide)	1m 3s
rpmbuild (x86_64, opensuse/leap:15)	1m 18s
rpmbuild (x86_64, opensuse/tumbleweed:latest)	47s
rpmbuild (ppc64le, almalinux:8)	2m 52s
rpmbuild (ppc64le, almalinux:9)	3m 35s
rpmbuild (ppc64le, fedora:37)	7m 30s
rpmbuild (ppc64le, fedora:38)	7m 10s
rpmbuild (ppc64le, fedora:rawhide)	5m 47s
rpmbuild (ppc64le, opensuse/leap:15)	5m 18s
rpmbuild (ppc64le, opensuse/tumbleweed:latest)	3m 2s
release	0s
	46m 54s

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