There is a known "xm console" issue related with VMX. When "serial" is

There is a known "xm console" issue related with VMX. When "serial" is
enabled in script and no once uses "xm console" to read the console,
VMX boting will hang due to the buffer is full.
I added a "select" before "write". If it could not be written,
unix_write will Return immediately and it will not block the VMX
booting. With this fix, we can  make VMX's serial enable by default.

Signed-off-by: Yu Ping <ping.y.yu@intel.com>
Modified to patch xmexample.hvm. Put through xenrt on a VMX box.

Signed-off-by: James Bulpin <james@xensource.com>