xend: Fix block device type check

xend: Fix block device type check

Since changeset 17617 the block device protocols are checked to be
either file or phy. This has later been fixed to also include
tap. This is still not a correct check as there can be arbitrary
additional protocols. Before this check was added you could have a
block-xyz hotplug script and xyz would work as a protocol, now it is
refused. An example for this mechanism is the block-nbd script which
is included in the tree.

This patch changes the check to allow file, phy, tap and all protocol
names for which a block-$protocol hotplug script exists. This should
fix the last regressions introduced by the check.

Signed-off-by: Kevin Wolf <kwolf@suse.de>