qemu: Use log output for pty assignment if 'info chardev' is unavailable

qemu: Use log output for pty assignment if 'info chardev' is unavailable

qemudFindCharDevicePTYsMonitor reports an error if 'info chardev' didn't
provide information for a requested device, even if the log output parsing
had found the pty path for that device. This makes pty assignment fail for
older QEMU/KVM versions. For example KVM 72 on Debian doesn't support
'info chardev', so qemuMonitorTextGetPtyPaths cannot parse any useful
information and the hash for device-id-to-pty-path mapping stays empty.

Make qemudFindCharDevicePTYsMonitor report an error only if the log output
parsing and the 'info chardev' parsing failed to provide the pty path.