x86/svm: Fix handling of ICEBP intercepts

x86/svm: Fix handling of ICEBP intercepts

c/s 9338a37d "x86/svm: implement debug events" added support for introspecting
ICEBP debug exceptions, but didn't account for the fact that
svm_get_insn_len() (previously __get_instruction_length) can fail and may
already have raised #GP with the guest.

If svm_get_insn_len() fails, return back to guest context rather than
continuing and mistaking a trap-style VMExit for a fault-style one.

Spotted by Coverity.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Razvan Cojocaru <rcojocaru@bitdefender.com>
Acked-by: Brian Woods <brian.woods@amd.com>