target-i386: SVM: acknowledge interrupt only after it is taken

SVM specifies that the V_IRQ mask is only to be removed, if the
interrupt that is to be delivered actually is delivered.

As of the SVM rewrite, this mask is always unmasked when the main cpu
loop is processed, leaving a corner case where calling the interrupt
handler causes a #PF. In that case (booting Linux / starting gfxboot)
the current implementation tells the VMM the interrupt is taken, even
though it is not.

This patch modifies the VIRQ unmasking to occur after do_interrupt,
making gfxboot work again.

Signed-off-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@6008 c046a42c-6fe2-441c-8c8c-71466251a162