x86: split Dom0 build into PV and PVHv2

x86: split Dom0 build into PV and PVHv2

Split the Dom0 builder into two different functions, one for PV (and classic
PVH), and another one for PVHv2. Introduce a new command line parameter called
'dom0' that can be used to request the creation of a PVHv2 Dom0 by setting the
'hvm' sub-option. A panic has also been added if a user tries to use dom0=hvm
until all the code is in place, then the panic will be removed.

While there mark the dom0_shadow option that was used by PV Dom0 as deprecated,
it was lacking documentation and was not functional. Point users towards
dom0=shadow instead.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>