boot: Support "halt" in the boot order to prevent default boot attempts.

Current seabios will attempt to boot from both configured devices and
a list of default devices.  Sometimes it is desirable to only attempt
to boot from the configured devices and not from any of the default
devices.  This patch adds support for a "HALT" item in the "bootorder"
file - its presence will prevent SeaBIOS from attempting any default
boot actions.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Signed-off-by: Amos Kong <akong@redhat.com>
vgabios: implement AX=1120H..1124H functions

These function only have to set INT 1Fh and INT 43h, and set
the BDA height + number of rows.

I could not find out whether AX=1120h should also set the character
height to 8.  I think not, because INT 43h might still point to
14- or 16-pixel high characters and in this case INT 1Fh will not
be used at all.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>