This patch introduces libxl_primary_console_exec: a new libxl function

This patch introduces libxl_primary_console_exec: a new libxl function
that finds the domid and console number corresponding to the primary
console of a given vm. The domid might be different from the domid of
the VM and the console number might not be 0 when using stubdoms.
The caller (xl_cmdimpl.c in this case) has to make sure that the stubdom
is already created before calling libxl_primary_console_exec in the hvm
case. In the PV case libxl_primary_console_exec has to be called before
libxl_run_bootloader.

Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>