librbd/migration: move away from util::create_ioctx() in NativeFormat

librbd/migration: move away from util::create_ioctx() in NativeFormat

This is another step towards supporting migration from external
clusters, where creating an IoCtx from a Rados instance that has
nothing to do with dst_io_ctx would be needed.  It also allows to
get rid of a pool lookup in the middle of parsing code.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 7986662157491419d366de3dd0070553abb9e1c8)

Conflicts:
	src/librbd/migration/NativeFormat.cc [ commit 8e58a52a408b
	  ("librbd/migration: don't include ImageState.h in formats")
	  was backported to squid out of order ]