libxl, xl: exec xenconsole in current process, defer decision to fork to caller

libxl, xl: exec xenconsole in current process, defer decision to fork to caller

Use this to run xenconsole as the foreground process and move the
connection to the console in the "create -c" case early enough to be
able to view output from the bootloader. This behaviour is consistent
with how both "xm console" and "xm create -c" operate.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>