Specifying an unsupported disk format with the tap driver resulted in
a less than helpful error message

error: Failed to start domain test-hvm
error: internal error libxenlight does not support disk driver qed

Change the message to state that the qed format is not supported by
the tap driver, e.g.

error: Failed to start domain test-hvm
error: internal error libxenlight does not support disk format qed
with disk driver tap

While at it, check for unsupported formats in the other driver
backends.