plugins/execlog: fix execlog vcpu_exit execution print loss

Executed instructions are cached in string format inside the
execlog plugin. These strings are flushed on exit of a TB, improving
performance. This causes executed instructions to be lost when an
'ecall' (riscv system call) occurs that causes the thread to terminate.

The fix in this patch registers a 'vcpu_exit' callback, and flushes
any content in the c->last_exec buffer, to ensure all instructions are
present in the final instruction log.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Tested-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>
Signed-off-by: Harry van Haaren <harry.vanhaaren@openchip.com>
Link: https://lore.kernel.org/qemu-devel/20260716094126.787556-3-harry.vanhaaren@openchip.com
Signed-off-by: Pierrick Bouvier <pierrick.bouvier@oss.qualcomm.com>