OvmfPkg/QemuFlashFvbServicesRuntimeDxe: correct NumOfLba vararg type in EraseBlocks()

OvmfPkg/QemuFlashFvbServicesRuntimeDxe: correct NumOfLba vararg type in EraseBlocks()

According to the PI spec, Volume 3,
EFI_FIRMWARE_VOLUME_BLOCK2_PROTOCOL.EraseBlocks():

> The variable argument list is a list of tuples. Each tuple describes a
> range of LBAs to erase and consists of the following:
> * An EFI_LBA that indicates the starting LBA
> * A UINTN that indicates the number of blocks to erase

(NB, in edk2, EFI_FIRMWARE_VOLUME_BLOCK_PROTOCOL is a typedef to
EFI_FIRMWARE_VOLUME_BLOCK2_PROTOCOL.)

In this driver, the NumOfLba local variable is defined with type UINTN,
but the TYPE argument passed to VA_ARG() is UINT32. Fix the mismatch.

Cc: Jordan Justen <jordan.l.justen@intel.com>
Reported-by: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>