Build fixes for various guests

The 2 patches that follow fix up various build issues I found while trying to build PH-on-HVM drivers against unstable.

[PATCH 1/2][PV-on-HVM] Fix compile errors for RHEL/SLES
Fix compile errors on a couple of distros.
1st fix was for SLES10-SP1 and SLES10-SP2:
netif_tx_lock_bh was pulled back into older kernels, and thus must be detected.
2nd fix was for RHEL4-U5 which pulled back the irqreturn_t feature into an older kernel. This too must be detected. 3rd fix was fixing a compile error in a printk for a more strict compile in RHEL4-U5

[PATCH 2/2][linux-2.6.18-xen][PV-on-HVM] Fix compile errors for RHEL/SLES Change unsigned to unsigned long for spinlock flags.
This avoids x86_64 assembly error in some distros (SLES10)