KVM: SVM: add helper functions for global interrupt flag

This patch makes the code easier to read when it comes to setting,
clearing and checking the status of the virtualized global
interrupt flag for the VCPU.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>