Xen PCI passthrough support may not be available and thus the global
variable "has_igd_gfx_passthru" might be compiled out. Common code
should not access it in that case.

Unfortunately, we can't use CONFIG_XEN_PCI_PASSTHROUGH directly in
xen-common.c so this patch instead move access to the
has_igd_gfx_passthru variable via function and those functions are
also implemented as stubs. The stubs will be used when QEMU is built
without passthrough support.

Now, when one will want to enable igd-passthru via the -machine
property, they will get an error message if QEMU is built without
passthrough support.

Fixes: 46472d82 ('xen: convert "-machine igd-passthru" to an accelerator property')
Reported-by: NRoger Pau Monné <roger.pau@citrix.com>
Signed-off-by: NAnthony PERARD <anthony.perard@citrix.com>
Message-Id: <20200603160442.3151170-1-anthony.perard@citrix.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>