efi: Only scan the BLKIO MEDIA once

Scan only the BLOCK IO MEDIA once instead of each time for each type of
device (fd, cd and hdd).
Leave the mechanism to free and reprobe all devices if one day we want
to implement a "dev rescan" thing.

Reviewed by: imp, tsoome
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D14334