Because trace_opt_parse always deletes the options it has parsed,
trace_init_file's call to qemu_find_opts_singleton always
creates an empty -trace option group.  Therefore, the subsequent
qemu_opt_get(opts, "file") always returns NULL.

To fix this, save the last "-trace file=..." option in a global
variable and use it later in trace_init_file.

This is similar to what was done before commit 92eecfff ("trace:
remove argument from trace_init_file", 2020-11-11), except contained
within trace/control.c and without memory leaks.

Fixes: 92eecfff ("trace: remove argument from trace_init_file", 2020-11-11)
Cc: stefanha@redhat.com
Reported-by:  <armbru@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20210209145759.141231-2-pbonzini@redhat.com>