x86/setup: Do not reserve a crash kernel region if booted on Xen PV

Xen PV domains cannot shut down and start a crash kernel. Instead,
the crashing kernel makes a SCHEDOP_shutdown hypercall with the
reason code SHUTDOWN_crash, cf. xen_crash_shutdown() machine op in
arch/x86/xen/enlighten_pv.c.

A crash kernel reservation is merely a waste of RAM in this case. It
may also confuse users of kexec_load(2) and/or kexec_file_load(2).
When flags include KEXEC_ON_CRASH or KEXEC_FILE_ON_CRASH,
respectively, these syscalls return success, which is technically
correct, but the crash kexec image will never be actually used.

Signed-off-by: Petr Tesarik <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Cc: Tom Lendacky <[email protected]>
Cc: Dou Liyang <[email protected]>
Cc: Mikulas Patocka <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: [email protected]
Cc: Boris Ostrovsky <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Jean Delvare <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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