metaflow: Convert hex parsing to use new utility functions.

metaflow: Convert hex parsing to use new utility functions.

We now have functions that can do parsing and printing of long hex
strings, so we should use them for meta flow fields to ensure
consistent behavior.

Since these functions can handle infinitely long strings, we can
also increase the maximum field size for MFS_HEXADECIMAL types to
the limit allowed by NXM/OXM. This is useful for future large fields,
such as Geneve options.

Signed-off-by: Jesse Gross <jesse@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>