CI: Rework domU_config generation in qubes-x86-64.sh

CI: Rework domU_config generation in qubes-x86-64.sh

Right now, various blocks rewrite domU_config= as a whole, even though it is
largely the same.

 * dom0pvh-hvm does nothing but change the domain type to hvm
 * *-pci sets the domain type, clears vif=[], appends earlyprintk=xen to the
   cmdline, and adds some PCI config.

Refactor this to be domU_type (defaults to pvh), domU_vif (defaults to
xenbr0), and domU_extra_config (defaults to empty) and use these variables to
build domU_config= once.

Of note, the default domU_config= now sets cmdline=, and extra= is intended
for inclusion via domU_extra_config as necessary.

No practical change.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Reviewed-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>