Several minor changes to hopefully complete the VirtIO console driver

  - Support the KDB alt break sequence to enter the debugger,
    panic, reboot, etc. [1]
  - Provide emergency write feature description. Note that QEMU
    does not implement this feature.
  - Make the VTCON_FLAG_* defines sequential once again.
  - When the multiple port feature is not negotiated, query the
    rows and columns of the one console during the device attach
    when the size feature is negotiated.
  - Report failure to the device if hot plugging a port fails.
  - Acknowledge the console port event with an open event. This
    is required by the spec, but QEMU doesn't seem to care.

Submitted by:	Juniper [1]
MFC after:	1 month


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274215 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f