savevm: Only qemu_fflush() can generate errors

Move the error check to the beggining of the callers.  Once this is fixed
qemu_file_set_if_error() is not used anymore, so remove it.

Signed-off-by: Juan Quintela <quintela@redhat.com>

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
