Fix cdrom eject

Cdrom eject intended to use POST "/vms/vm-name/vmstorages/hdc" to eject
cdrom, while UI used PUT "/vms/vm-name/vmstorages/hdc" to implement this,
which makes cdrom eject not working.
Adjust backend code to fix this and delete eject action.

Signed-off-by: Royce Lv <lvroyce@linux.vnet.ibm.com>