char: fix parameter name / type in BSD codepath

The BSD impl of qemu_chr_open_pp_fd had mis-declared
its parameter type as ChardevBackend instead of
ChardevCommon. It had also mistakenly used the variable
name 'common' instead of 'backend'.

Tested-by: Sean Bruno <sbruno@freebsd.org>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>