ipmi: check return of qemu_chr_fe_write() for errors

The continue_send() method in ipmi_bmc_extern.c directly
assigns the return value of qemu_chr_fe_write() to the
variable tracking the I/O buffer offset. This ignores the
possibility that the return value could be -1 and so will
cause I/O go backwards on EAGAIN. Fortunately 'outpos' is
unsigned, so can't go negative - it will become MAX_INT
which will cause the loop to stop, and avoid an accidental
out of bounds array access.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <1473170165-540-2-git-send-email-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
