lguest: Read length of device_cap later

Read the length of the capability with type VIRTIO_PCI_CAP_DEVICE_CFG
only when we're sure we're going to need it. Which is just before the
check whether the virtio console actually has an emerg_wr field.

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: lguest@lists.ozlabs.org
Link: http://lkml.kernel.org/r/1457006501-5377-2-git-send-email-pebolle@tiscali.nl
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>