lguest: Read offset of device_cap later

Read the offset of the capability with type VIRTIO_PCI_CAP_DEVICE_CFG
only when we're sure we're going to need it. Which is when all checks
have passed and we know we have a virtio console with 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/1457006501-5377-3-git-send-email-pebolle@tiscali.nl
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>