recovery: ignore voldmanaged volumes

Voldmanaged volumes use sysfs path instead of device
path which is not supported by recovery, so a separate
entries for /sdcard and /external_sd are required.

We ignore these volume so we don't show the menu for
"mount /storage/sdcard0", etc.

Change-Id: I6bf26e0ad3fb59579c986cf5537f9e70448891e9