vtpmmgr: properly remove t_uint size dependency

vtpmmgr: properly remove t_uint size dependency

Rather than using the internal MPI format for the Diffie-Hellman group,
whose representation depends on the size of the t_uint type, store the
value as a big-endian integer and use mpi_read_binary to convert it in
an architecture-independent manner.  This patch also removes the
unnecessary range check on the exponent which ended up being different
between 32- and 64-bit code.

Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Acked-by: Ian Campbell <ian.campbell@citrix.com>