Qemu on Windows fails to parse absolute file path in -acpitable switch
Description of problem:
I expect qemu-system-x86_64.exe to navigate to the path provided with -acpitable switch and to try to parse it. Instead, Qemu prints: "can't open file C: No such file or directory" if provided with absolute path. Qemu thinks "C:" itself is a file with acpi table.

However, Qemu correctly processes files with relative path. If I run this command to try to parse file COPYING bundled in default qemu build:

`qemu-system-x86_64.exe -acpitable "file=copying"`

Qemu says: `qemu-system-x86_64.exe: -acpitable file=copying: warning: ACPI table has wrong length, header says 1313284128, actual size 17992 bytes`

Then it proceeds to boot BIOS, as usual.
Steps to reproduce:
1. Run `qemu-system-x86_64.exe -acpitable "file=C:\temp\temp.txt"`
2. Experience "can't open file C: No such file or directory" error message returning you to the command prompt. No BIOS screen.
3. Run `qemu-system-x86_64.exe -acpitable "file=copying"` 
4. Experience insignificant warning and then a normal BIOS screen.