[efi] Install our own disk I/O protocol and claim exclusive use of it

[efi] Install our own disk I/O protocol and claim exclusive use of it

The EFI FAT filesystem driver has a bug: if a block device contains no
FAT filesystem but does have an EFI_SIMPLE_FILE_SYSTEM_PROTOCOL
instance, the FAT driver will assume that it must have previously
installed the EFI_SIMPLE_FILE_SYSTEM_PROTOCOL.  This causes the FAT
driver to claim control of our device, and to refuse to stop driving
it, which prevents us from later uninstalling correctly.

Work around this bug by opening the disk I/O protocol ourselves,
thereby preventing the FAT driver from opening it.

Note that the alternative approach of opening the block I/O protocol
(and thereby in theory preventing DiskIo from attaching to the block
I/O protocol) causes an endless loop of calls to our DRIVER_STOP
method when starting the EFI shell.  I have no idea why this is.

Signed-off-by: Michael Brown <mcb30@ipxe.org>