XSTS Cli fixes

[mondokm]

This PR adds a different default solver for CHC and fixes cex length printing in XSTS Cli

[github-actions]

Benchexec test report for a selection of SV-Benchmarks (correct / incorrect / all):

task set	BOUNDED	CEGAR	HORN
ConcurrencySafety-Main	❓ (0 / 0 / 50) HTML/CSV	✅ (20 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV
ConcurrencySafety-MemSafety	❓ (0 / 0 / 50) HTML/CSV	✅ (43 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV
ConcurrencySafety-NoOverflows	❓ (0 / 0 / 50) HTML/CSV	✅ (39 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV
NoDataRace-Main	❓ (0 / 0 / 50) HTML/CSV	✅ (26 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV
ReachSafety-Arrays	❓ (0 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV
ReachSafety-BitVectors	✅ (14 / 0 / 46) HTML/CSV	✅ (13 / 0 / 46) HTML/CSV	✅ (12 / 0 / 46) HTML/CSV
ReachSafety-Combinations	❓ (0 / 0 / 50) HTML/CSV	✅ (3 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV
ReachSafety-ControlFlow	✅ (3 / 0 / 50) HTML/CSV	✅ (2 / 0 / 50) HTML/CSV	✅ (17 / 0 / 50) HTML/CSV
ReachSafety-ECA	❓ (0 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV
ReachSafety-Floats	❗ (7 / 1 / 50) HTML/CSV	✅ (15 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV
ReachSafety-Hardware	✅ (2 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV
ReachSafety-Heap	✅ (2 / 0 / 11) HTML/CSV	✅ (10 / 0 / 11) HTML/CSV	❗ (2 / 1 / 11) HTML/CSV
ReachSafety-Loops	✅ (16 / 0 / 50) HTML/CSV	✅ (11 / 0 / 50) HTML/CSV	✅ (12 / 0 / 50) HTML/CSV
ReachSafety-Recursive	❓ (0 / 0 / 50) HTML/CSV	✅ (13 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV
ReachSafety-Sequentialized	❓ (0 / 0 / 50) HTML/CSV	✅ (1 / 0 / 50) HTML/CSV	✅ (2 / 0 / 50) HTML/CSV
ReachSafety-XCSP	❓ (0 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV	❓ (0 / 0 / 50) HTML/CSV