migration: Move channel parsing to channel.c

Encapsulate the MigrationChannelList parsing in a new
migrate_channels_parse() located at channel.c.

This also makes the memory management of the MigrationAddress more
uniform. Previously, half the parsing code (uri parsing) would
allocate memory for the address while the other half (channel parsing)
would instead pass the original QAPI object along. After this patch,
the MigrationAddress is always QAPI_CLONEd, so the callers can use
g_autoptr(MigrationAddress) in all cases.

Reviewed-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Prasad Pandit <pjp@fedoraproject.org>
Link: https://lore.kernel.org/qemu-devel/20260123141656.6765-21-farosas@suse.de
Signed-off-by: Fabiano Rosas <farosas@suse.de>