SVM: use generic instruction decoding

SVM: use generic instruction decoding

... instead of custom handling. To facilitate this break out init code
from _hvm_emulate_one() into the new hvm_emulate_init(), and make
hvmemul_insn_fetch( globally available.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>