[cpuid] Allow reading hypervisor CPUID leaves

[cpuid] Allow reading hypervisor CPUID leaves

Hypervisors typically intercept CPUID leaves in the range 0x40000000
to 0x400000ff, with leaf 0x40000000 returning the maximum supported
function within this range in register %eax.

iPXE currently masks off bit 30 from the requested CPUID leaf when
checking to see if a function is supported, which causes this check to
read from leaf 0x00000000 instead of 0x40000000.

Fix by including bit 30 within the mask.

Signed-off-by: Michael Brown <mcb30@ipxe.org>