kvm_main: Use common error handling code in kvm_dev_ioctl_create_vm()

Add a jump target so that a bit of exception handling can be better reused
at the end of this function.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>