Fix FlagValue dump behavior.

When running the command method on a packet, by default, the __repr__
method of the value is used to dump a field.
Although, the __repr__ method of the new FlagValue class is not a valid
python syntax:

In [2]: IP(flags=0).command()
Out[2]: 'IP(flags=<Flag 0 ()>)'

In [3]: IP(flags=<Flag 0 ()>)
  File "<ipython-input-3-1e8a4a1eb25a>", line 1
    IP(flags=<Flag 0 ()>)
             ^
SyntaxError: invalid syntax

Also, when print a FlagValue with value of 0, flagrepr returns an empty
string which can be confusing:

In [1]: IP(flags=0)
Out[1]: <IP  flags= |>

Use int value instead in both cases.

Fixes: fc6a4caa ("Introduce FlagValue(int) objects to represent FlagsField() values")
Signed-off-by: Thomas Faivre <thomas.faivre@6wind.com>