qga: fix erroneous argument to strerror

process_command returns a negative value in case of error.  Make this
clear in the "if" statement and fix the strerror argument to flip it
to positive.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
