app/testpmd: support external memory

app/testpmd: support external memory

Currently, mempools can only be allocated either using native
DPDK memory, or anonymous memory. This patch will add two new
methods to allocate mempool using external memory (regular or
hugepage memory), and add documentation about it to testpmd
user guide.

It adds a new flag "--mp-alloc", with four possible values:
native (use regular DPDK allocator), anon (use anonymous
mempool), xmem (use externally allocated memory area), and
xmemhuge (use externally allocated hugepage memory area). Old
flag "--mp-anon" is kept for compatibility.

All external memory is allocated using the same external heap,
but each will allocate and add a new memory area.

Suggested-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>
Acked-by: Bernard Iremonger <bernard.iremonger@intel.com>