Ignore IDE command if issued while IDE is busy (Gleb Natapov)

Ignore IDE command if issued while IDE is busy (Gleb Natapov)

Feature, Sector Count, LBA Low/Mid/High and Device registers should be
written only when both BSY and DRQ are cleared to zero.
Command register shall only be written when BSY and DRQ are set to zero
for all commands except DEVICE RESET.
Data Port register shall be accessed for host PIO data transfer only when
DRQ is set to one.

Signed-off-by: Gleb Natapov <gleb@qumranet.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5060 c046a42c-6fe2-441c-8c8c-71466251a162