qemu, lxc, lxd: Try to use a user account in the 1000-59999 range

Some autopkgtests, such as the ones for dbus, require an ordinary user account with a non-trivial home directory. Debian Policy says we are most likely to find such accounts in the 1000-59999 range, and the vmdebootstrap invocation suggested in autopkgtest-virt-qemu(1) creates one.

These virt providers historically looked for uids >= 500 because some non-Debian-derived distributions started allocating users at 500, which was usually OK, because adduser --system creates system users in the range 100-999, starting from the bottom and working upwards.

However, some system users for systemd daemons are now allocated dynamically by systemd-sysusers, which allocates uids in the system range 100-999 from the top down: on my test VM, the offending user account was systemd-coredump, which is used by systemd-coredump but currently created by /usr/lib/sysusers.d/systemd.conf in systemd. This is not a suitable account for automated testing, because it cannot write to its home directory '/' and has the nologin shell.

Signed-off-by: Simon McVittie smcv@debian.org

Closes: #897170