OvmfPkg/QemuVideoDxe: Shouldn't assume system in VGA alias mode.

OvmfPkg/QemuVideoDxe: Shouldn't assume system in VGA alias mode.

Query the supported attributes firstly, then bitwise AND (&) both VGA_IO
and VGA_IO_16. Since the supported attributes should only have one of
VGA_IO or VGA_IO_16 set, the result of bitwise AND (&) is either VGA_IO
or IO_16. Then the result can be passed to PciIo->Attributes() to set the
attributes.

Device driver should consider both since the mReserveVgaAliases in
PciBusDxe driver is default FALSE(implies that device driver can only set
VGA_IO_16 to PCI_ROOT_BRIDGE), and Platform code may not return
EFI_RESERVE_VGA_IO_ALIAS in GetPlatformPolicy of PciPlatformProtocol to
make mReserveVgaAliases become TRUE(implies that device driver can only
set VGA_IO to PCI_ROOT_BRIDGE), Currently OvmfPkg doesn't have problem
due to it has hard code value for PCI_ROOT_BRIDGE's attributes field, so
an IO access by PciIoProtocol will be successed due to
RootBridgeIoCheckParameter of PciRootBridgeIo.c will always get pass
result for legacy IO access.

Usually the attributes field of PCI_ROOT_BRIDGE should be 0, in that case
it will have issue since the VGA_IO may not be able to be enabled, then
IO access by PciIoProtocol will be failed, hence the QemuVideoDxe driver
will not work fine.

Signed-off-by: Marc Chen <marc.w.chen@intel.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Anthony Perard <anthony.perard@citrix.com>
Cc: Julien Grall <julien.grall@arm.com>
Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Stefan Berger <stefanb@linux.ibm.com>
Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=1880
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Message-Id: <20190606074237.81492-1-marc.w.chen@intel.com>
Signed-off-by: Laszlo Ersek <lersek@redhat.com>