xen: Simplify halting of first CPU

Use the global first_cpu variable to halt the CPU rather than using a
local first_cpu initialized from qemu_get_cpu(0).

This will allow to change qemu_get_cpu() return type to CPUState
despite use of the CPU_COMMON halted field in the reset handler.

Signed-off-by: Andreas Färber <afaerber@suse.de>
