Contain MSI errors in msi emulation code

Contain MSI errors in msi emulation code

If msi=0 is specified and guest want to enable msi, qemu finds it can
not enable it and quits.  (This problem is not appicable for xen
unstable since the "msi=x" switch is removed.)  This patch let these
cases be handled inside msi emulation code. If qemu can not enable
msi, guest will not receive MSI.  But Qemu can still go on.  But if
guest driver does not test MSI's functionality, guest device will stop
to function.

Submitted-By: Haitao Shan
Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>