qemu-vm: replace virtio_scsi_pci with generic scsi_hba denomination

There are other SCSI HBAs that virtio-scsi, and they all can be tested
in the same way.
guest-hw: split parts out into backend.cfg

Things such as image formats are not guest hardware

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
guest-os: move x86_64 vm_arch_name to the single files

Prepare for selecting the architecture name with an "only" directive.
guest-hw: add variants for vm_arch_name

Select the architecture name in guest-hw, so that we can
filter out unwanted tests from the test runner.
tests: add only directive for architectures

Some tests are x86-specific.  Use the new architecture variant to
filter them away.
runner: add --arch option

We can now tell autotest the architecture being tested, and use it to filter
unwanted tests.
guest-hw: add variants for machine_type

Also select the machine type in guest-hw.  This is also useful to specify
versioned machine types in a cartesian config file, using redefinitions
such as:

    mach_i440fx:
         variants:
         - i440fx-1.0:
              machine_type=pc-1.0
         - i440fx-1.1:
              machine_type=pc-1.1
         - i440fx-1.2:
              machine_type=pc-1.2
cfg: add only directives for machine types
runner: add --machine-type option
jeos: prepare for non-Fedora JeOS images

In principle, there is no reason why JeOS images should be Fedora images.
To prepare for that, let the config parser pick any JeOS image that
satisfies --arch and --machine-type requirements.  And move existing
Fedora-specific values to the JeOS.17.64 definition.
qemu: support NIC with no model

Some architectures do not support using -device to instantiate NICs.
Instead, you use "-net nic,netdev=foo" and the QEMU board code will
define the NICs.  These can be supported by not defining a value
for the nic_model value.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
guest-hw: add a couple more drive types

spapr_vscsi is used by ppc64.  sd is used on ARM and other embedded
architectures.