vl.c: Don't print errno after failed qemu_chr_new()

vl.c: Don't print errno after failed qemu_chr_new()

The qemu_chr_new() function doesn't set errno on failure, so
don't print strerror(errno) on the error handling path when
dealing with the -serial, -parallel and -virtioconsole arguments.
This avoids nonsensical error messages like:
  $ ./arm-softmmu/qemu-system-arm -serial wombat
  qemu: could not open serial device 'wombat': Success

We also rephrase the message slightly to make it a little clearer
that we're expecting the name of a QEMU chr backend rather than
a host or guest serial/parallel/etc device.

Reported-by: Christian Müller <christian.mueller@heig-vd.ch>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>