kvm tools: Fix formatting of error message in TAP handling

kvm tools: Fix formatting of error message in TAP handling

This error message is missing a space, and has a redundant ":" at the end,
currently it produces:

  You have requested a TAP device, but creation of one hasfailed because:: No such file or directory

Add a space to "hasfailed" and remove the extra ":".

Don't split the line to improve grepability.

Acked-by: Asias He <asias.hejun@gmail.com>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Pekka Enberg <penberg@kernel.org>