Remove useless codes for server_group

Some codes about 'extract_members' from 'create_server_group'
is written in original bp/patch's reviewing
(I650a8f191dea5eab5b4b1828f0b9f65e33edea2a).

But as the design variation, it can't be specified members
in create API (see it in PS29 vs PS30).
But some useless codes are still kept after this feature merged.
So it's necessary to remove them.

Change-Id: I087c8389f2c735a36d656b44c6a3ecfe3a4aa5a3
Closes-Bug: #1325148
libvirt: split and test finish_migration disk resize

Adds a '_disk_resize' method to help resizing disk
by checking and preparing the disk to be extend with disk.api.
Update finish_migration to use this helper.

Closes-Bug: #1298975
Signed-off-by: Sahid Orentino Ferdjaoui <sahid.ferdjaoui@cloudwatt.com>

Change-Id: Iefe16915a2e657d848629d7e4a4fcf67298f913e
Merge "Remove useless codes for server_group"
Merge "libvirt: split and test finish_migration disk resize"