xen/arm: vpl011: Define common ring buffer helper functions in console.h
DEFINE_XEN_FLEX_RING(xencons) defines common helper functions such as
xencons_queued() to tell the current size of the ring buffer,
xencons_mask() to mask off the index, which are useful helper functions.
pl011 emulation code will use these helper functions.
io/console.h includes io/ring.h which defines DEFINE_XEN_FLEX_RING.
In console/daemon/io.c, string.h had to be included before io/console.h
because ring.h uses string functions.