Call per-device post-parse callback even on implicit video

Call per-device post-parse callback even on implicit video

Commit 6879be48 moved adding of an implicit video device after XML
parsing. As a result, libxlDomainDeviceDefPostParse() is no longer
called to set the default vram when adding an implicit device.
Commit 6879be48 assumes virDomainVideoDefaultRAM() will set the
default vram, but it returns 0 if the domain virtType is
VIR_DOMAIN_VIRT_XEN. Attempting to start an HVM domain with vram=0
results in

error: unsupported configuration: videoram must be at least 4MB for CIRRUS

The default vram setting for Xen HVM domains depends on the device
model used (qemu-xen vs qemu-traditional), hence setting the
default is deferred to libxlDomainDeviceDefPostParse().

Call the device post-parse callback even for implicit video,
to fill out the default vram even for VIR_DOMAIN_VIRT_XEN.

https://bugzilla.redhat.com/show_bug.cgi?id=1334557
Most-of-commit-message-by: Jim Fehlig <jfehlig@suse.com>