xen: Code cleanup and small bug fixes

xen: Code cleanup and small bug fixes

xen/hypervisor.h:
 - Remove unused helpers: MULTI_update_va_mapping, is_initial_xendomain,
   is_running_on_xen
 - Remove unused define CONFIG_X86_PAE
 - Remove unused variable xen_start_info: note that it's used inpcifront
   which is not built at all
 - Remove forward declaration of HYPERVISOR_crash

xen/xen-os.h:
 - Remove unused define CONFIG_X86_PAE
 - Drop unused helpers: test_and_clear_bit, clear_bit,
   force_evtchn_callback
 - Implement a generic version (based on ofed/include/linux/bitops.h) of
   set_bit and test_bit and prefix them by xen_ to avoid any use by other
   code than Xen. Note that It would be worth to investigate a generic
   implementation in FreeBSD.
 - Replace barrier() by __compiler_membar()
 - Replace cpu_relax() by cpu_spinwait(): it's exactly the same as rep;nop
   = pause

xen/xen_intr.h:
 - Move the prototype of xen_intr_handle_upcall in it: Use by all the
   platform

x86/xen/xen_intr.c:
 - Use BITSET* for the enabledbits: Avoid to use custom helpers
 - test_bit/set_bit has been renamed to xen_test_bit/xen_set_bit
 - Don't export the variable xen_intr_pcpu

dev/xen/blkback/blkback.c:
 - Fix the string format when XBB_DEBUG is enabled: host_addr is typed
   uint64_t

dev/xen/balloon/balloon.c:
 - Remove set but not used variable
 - Use the correct type for frame_list: xen_pfn_t represents the frame
   number on any architecture

dev/xen/control/control.c:
 - Return BUS_PROBE_WILDCARD in xs_probe: Returning 0 in a probe callback
   means the driver can handle this device. If by any chance xenstore is the
   first driver, every new device with the driver is unset will use
   xenstore.

dev/xen/grant-table/grant_table.c:
 - Remove unused cmpxchg
 - Drop unused include opt_pmap.h: Doesn't exist on ARM64 and it doesn't
   contain anything required for the code on x86

dev/xen/netfront/netfront.c:
 - Use the correct type for rx_pfn_array: xen_pfn_t represents the frame
   number on any architecture

dev/xen/netback/netback.c:
 - Use the correct type for gmfn: xen_pfn_t represents the frame number on
   any architecture

dev/xen/xenstore/xenstore.c:
 - Return BUS_PROBE_WILDCARD in xctrl_probe: Returning 0 in a probe callback
   means the driver can handle this device. If by any chance xenstore is the
  first driver, every new device with the driver is unset will use xenstore.

Note that with the changes, x86/include/xen/xen-os.h doesn't contain anymore
arch-specific code. Although, a new series will add some helpers that differ
between x86 and ARM64, so I've kept the headers for now.

Submitted by:		Julien Grall <julien.grall@citrix.com>
Reviewed by:		royger
Differential Revision:	https://reviews.freebsd.org/D3921
Sponsored by:		Citrix Systems R&D