x86/event: correct debug event generation

x86/event: correct debug event generation

RIP is not a linear address, and hence should not on its own be subject
to GVA -> GFN translation. Once at it, move all of the (perhaps
expensive) operations in the two functions into their main if()'s body,
and improve the error code passed to the translation function.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>