exec.c: check memory backend file size with 'size' option

If the memory backend file is not large enough to hold the required 'size',
Qemu will report error and exit.

Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
Message-Id: <20161027042300.5929-3-haozhong.zhang@intel.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20161102010551.2723-1-haozhong.zhang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
