fix(engine,stats): add libvirt overload protection with timeouts and backoff

Engine changes:

Add update_hyp_cap_status() to disable/enable capabilities independently
Add timeout protection to isAlive() check (5s timeout)
Disable capabilities when libvirt is unresponsive, re-enable on recovery
Block UI events enqueuing when hypervisor capabilities are disabled
Add detailed logging with LIBVIRT prefixes for monitoring

Stats changes:

Fix domain resource leak (uncommented Free() calls)
Add exponential backoff (2s to 5min) for libvirt failures
Add timeout wrappers for ListAllDomains and GetAllDomainStats (10s)
Reduce MaxRequestsInFlight from 40 to 5 to prevent mutex contention
Add detailed logging with LIBVIRT prefixes for monitoring

Log prefixes for grep monitoring:

LIBVIRT TIMEOUT: Operation exceeded timeout
LIBVIRT SLOW: Operation completed but slow
LIBVIRT BACKOFF INCREASED: Backoff increased after failure
LIBVIRT RECOVERED: System recovered after failures