char: allocate CharDriverState as a single object

char: allocate CharDriverState as a single object

Use a single allocation for CharDriverState, this avoids extra
allocations & pointers, and is a step towards more object-oriented
CharDriver.

Gtk console is a bit peculiar, gd_vc_chr_set_echo() used to have a
temporary VirtualConsole to save the echo bit. Instead now, we consider
whether vcd->console is set or not, and restore the echo bit saved in
VCDriverState when calling gd_vc_vte_init().

The casts added are temporary, they are replaced with QOM type-safe
macros in a later patch in this series.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>