target-i386: Fix vm86 mode regression introduced in fd460606.

Commit fd460606 moved setting of eflags above calls to
cpu_x86_load_seg_cache() in seg_helper.c.  Unfortunately, in
do_interrupt_protected() this moved the clearing of VM_MASK above a
test for it.

Fix this regression by storing the value of VM_MASK at the start of
do_interrupt_protected().

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>