tracetool: fix "PRI" macro decoding

tracetool: fix "PRI" macro decoding

macro is not reset after use, so the format decoded is always the
one of the first "PRI" in the format string.

For instance:

  vhost_vdpa_set_config(void *dev, uint32_t offset, uint32_t size, \
                        uint32_t flags) "dev: %p offset: %"PRIu32" \
                        size: %"PRIu32" flags: 0x%"PRIx32

generates:

  printf("%d@%d vhost_vdpa_set_config dev: %p offset: %u size: %u \
          flags: 0x%u\n", pid(), gettimeofday_ns(), dev, offset, \
          size, flags)

for the "flags" parameter, we can see a "0x%u" rather than a "0x%x"
because the first macro was "PRIu32" (for offset).

In the loop, macro becomes "PRIu32PRIu32PRIx32", and c_macro_to_format()
returns always macro[3] ('u' in this case). This patch resets macro after
the format has been decoded.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-id: 20210105191721.120463-3-lvivier@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>