9pfs: improve v9fs_open() tracing

9pfs: improve v9fs_open() tracing

Improve tracing of 9p 'Topen' request type by showing open() flags as
human-readable text.

E.g. trace output:

  v9fs_open tag 0 id 12 fid 2 mode 100352

would become:

  v9fs_open tag=0 id=12 fid=2 mode=100352(RDONLY|NONBLOCK|DIRECTORY|
  TMPFILE|NDELAY)

Therefor add a new utility function qemu_open_flags_tostr() that converts
numeric open() flags from host's native O_* flag constants to a string
presentation.

9p2000.L and 9p2000.u protocol variants use different numeric 'mode'
constants for 'Topen' requests. Instead of writing string conversion code
for both protocol variants, use the already existing conversion functions
that convert the mode flags from respective protocol constants to host's
native open() numeric flag constants and pass that result to the new
string conversion function qemu_open_flags_tostr().

Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <E1tTgDR-000oRr-9g@kylie.crudebyte.com>