Add DEV_XENTIMER option

[kvinupriya_gmail.com]

Summary

Check for DEV_XENTIMER in pv.c as xen_timer.c code that implements
the xen_clock_init is under "options xentimer" in the files file.

Sponsored by: Juniper Networks, Inc.

[ehem_freebsd_m5p.com]

Looks reasonable to me. Though the option is primarily to allow removing the timer on ARM/RISC-V (where it doesn't presently work). I'm unsure whether it is possible to avoid on x86.

[royger]

sys/x86/xen/pv.c

111

Won't this lead to a call into NULL when booted as Xen PVH guest but without DEV_XENTIMER enabled? See clock_init().

You need an else branch here to define it to the native function (native_clock_source_init()), or otherwise patch the callers to only call early_clock_source_init if != NULL.