Fix x86 feature modifications for features that set multiple bits

Fix x86 feature modifications for features that set multiple bits

QEMU allows adding or removing cpu features by using the syntax '-cpu +feature'
or '-cpu -feature'.  Some cpuid features cause more than one bit to be set or
cleared; but QEMU stops after just one bit has been modified, causing the
feature bits to be inconsistent.

Fix by allowing all feature bits corresponding to a given name to be set.

Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>