xen: add and enable Xen console for PVH guests

This adds and enables the PV console used on XEN kernels to
GENERIC/XENHVM kernels in order for it to be used on PVH.

Approved by: gibbs
Sponsored by: Citrix Systems R&D

dev/xen/console/console.c:

• Define console_page.
• Move xc_printf debug function from i386 XEN code to generic console code.
• Rework xc_printf.
• Use xen_initial_domain instead of open-coded checks for Dom0.
• Gate the attach of the PV console to PV(H) guests.

dev/xen/console/xencons_ring.c:

• Allow the PV Xen console to output earlier by directly signaling the event channel in start_info if the event channel is not yet initialized.
• Use HYPERVISOR_start_info instead of xen_start_info.

i386/include/xen/xen-os.h:

• Remove prototype for xc_printf since it's now declared in global xen-os.h

i386/xen/xen_machdep.c:

• Remove previous version of xc_printf.
• Remove definition of console_page (now it's defined in the console itself).
• Fix some printf formatting errors.

x86/xen/pv.c:

• Add some early boot debug messages using xc_printf.
• Set console_page based on the value passed in start_info.

xen/xen-os.h:

• Declare console_page and add prototype for xc_printf.