trace: fix --trace-backtraces uninitialized data in backtrace

Also fix other review comments related to 1f161695d86d.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
gdb: extract symbol/file/line resolution into a class

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
gdb: cache syminfo objects

Looking up symbols is expensive, store them in a cache.

Speeds up dumping the trace log when a backtrace is taken.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
gdb: resolve backtrace symbols in trace logs

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>
Merge branch 'trace'

Some fixups to --trace-backtrace, plus symbol resolution in the log output.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>