i386: hvf: Don't miss 16-bit displacement

i386: hvf: Don't miss 16-bit displacement

In 16-bit addressing mode, when Mod = 0 and R/M = 6, decoded displacement
doesn't reach decode_linear_addr and gets lost. Instructions that
involve the combination of ModRM always get a pointer with zero offset
from the beginning of DS segment.

The change fixes drawing in F-BIRD from day 1 of '18 advent calendar.

Signed-off-by: Roman Bolshakov <[email protected]>
Message-Id: <20190125154743[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>