Add basic support for multipath block devices.  The "sanboot" and
"sanhook" commands now accept a list of SAN URIs.  We open all URIs
concurrently.  The first connection to become available for issuing
block device commands is marked as the active path and used for all
subsequent commands; all other connections are then closed.  Whenever
the active path fails, we reopen all URIs and repeat the process.

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

Add a dummy SAN device which allows the "sanhook" command to be tested
even when no SAN booting capability is present on the platform.  This
allows substantial portions of the SAN boot code to be run in Linux
under Valgrind.

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