Add "monitors config position" capability.
Add "monitors config position" capability.

This will allow Windows guests to determine if the client supports a
monitors_config message from a multi-monitor guest that is not
multi-headed, i.e., that has one monitor per driver.

It keeps commit:8b0cd321d5a4d08ccba5845c5f2206e6f8032c1d
from breaking if an updated win-qxl driver is paired with an older client.

This resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1248189
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Changed from v1: Removed a local commit that preceded this one.
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Changed from v2: Changed name to "monitors config position"
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SPICE/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk